celebrated person at an event.

The kola nut itself is filled with caffeine:

Kola nut, which contains high amounts of caffeine, helps combat fatigue and is most commonly used as a central nervous system stimulant that focuses on the cerebrospinal centers. It also contains theobromine, a stimulant found in chocolate as well as in green tea.

Offering kola nut at a meeting would be equivalent of being hospitable. If someone visited one, as a means of being hospitable, one would offer them coffee, tea, or soda. In Okonkwo's culture, showing hospitality meant offering kola nut and palm wine. Having good manners is showing one's guest hospitality. It is a necessary part of Okonkwo's respectful cultural traditions.

Also, no doubt the kola nut was used as a stimulant since is has caffeine. It would be used the way one uses coca cola. In fact, coca cola has properties of kola nuts:

Most people around the world are familiar with kola; many have tasted it and do not even know it. In the 1800s, a pharmacist in Georgia took extracts of kola, sugar and coca and mixed them with carbonated water. His accountant tasted it and called it "Coca Cola." Today, Coca-Cola still uses kola in its original recipe.

Most importantly, when entertaining, it is necessary to show good manners. It is a sign of respect. The proper custom is to show hospitality to a guest. In Okonkwo's culture, offering kola nuts and palm wine is the same as showing good manners and respect for the guest or guests.

Nigerian-born Chinua Achebe published his first novel, Things Fall Apart, in 1958. He has said that he wrote the novel primarily as an educational tool, though of course the work is fiction. He wanted to dispel the perception that tribal Africans were bloodthirsty savages who needed rescuing by white Europeans on some kind of a mission from God.

It is true that this chronicle of the Igbo tribe includes plenty of bloodshed, in-fighting, and other things modern white society would not generally consider acceptable (such as pluralism--men marrying more than one wife). What is also true, however, is that the Igbos are a people who have profound respect for the customs and traditions of their tribe.

One of those social customs is embodied in the kola nut, and an incident in chapter one of the novel depicts both the importance of the kola nut and the civility with which these tribe members conduct their affairs.

Unoka gets a visitor one day. His name is Okoye, and after they shake hands, Unoka goes into another room and returns with 

a small wooden disc containing a kola nut, some alligator pepper and a lump of white chalk.

He sits down proudly announces that he has kola and passes the disc to his friend. What follows is an almost comical exchange between the two men over which of them must have the honor of breaking the kola nut. 

Unoka is finally the one who breaks the nut, and it is clearly part of a ritual in which all items on the disc (the lump of white chalk and the alligator pepper) are used. As Unoka opens the kola nut, his guest takes the white chalk and draws some lines on the floor before painting his big toe white with chalk. 

This is the beginning of a time-honored ritual. It continues as follows:

As he broke the kola, Unoka prayed to their ancestors for life and health, and for protection against their enemies. When they had eaten they talked about many things: about the heavy rains which were drowning the yams, about the next ancestral feast and about the impending war with the village of Mbaino.

Clearly this ritual is well known and much-practiced, as both men know their roles without discussion or hesitation. We do not doubt that if Okoye had been the one selected to break the kola nut, nothing would have changed except that Okoye would have been the one with a white toenail.

This scene which features the kola nut depicts these two Igbo men as civilized beings rather than as savages who show little respect for guests or anyone else.

In chapter four, after Okonkwo beats his wife during Peace Week, the priest of Ezeani refuses to share hospitality with him. He says to Okonkwo:

“Take away your kola nut. I shall not eat in the house of a man who has no respect for our gods and ancestors."

Obviously Okonkwo's wicked actions have compromised his position in the tribe and he is no longer worthy of the honor conferred by the kola nut. This adds to the significance of the kola nut as being more than just a well known social custom; instead it is symbolic of the worth of all men until they prove themselves unworthy.

The Igbo believe that "[h]e who brings kola brings life." That philosophy is embodied in the actions of the priest, mentioned above. If kola brings life, it should not be shared by or given to those who do not promote life. Okonkwo is violent in many ways, and in this tribe, what one man does affects everyone. He is not worthy of the kola.

Achebe achieves his goal of showing the honor and civility of the Igbos through the kola nut.

The kola nut is frequently brought up in Achebe's novel Things Fall Apart.  It is an important piece of Ibo culture.  Among other things it is an important part of Ibo ceremonies, but not only used for ceremonial purposes.  The Ibo people would use the kola nut as a sign of hospitality to guests.  In the novel, Okoye goes to collect a debt from Unoka, but before the two men got down to the business at hand, Unoka offered his guest kola as a sign of his hospitality.  The two men couldn't decide who should receive the honor of breaking the kola nut, and that scene helped Achebe lay out to his readers the importance of the kola nut.  The breaking of the nut also has some spiritual connections for the Ibo people as well.  This is indicated by the Ibo praying to their ancestors while breaking the kola nut.