In terms of its plot, the Iliad is fairly simple. It is the story of the Achaians (Greeks) and their war against the Trojans. It looks at the causes of the war and at the way the war went.
But the plot is not what is truly important about this poem. Instead, its greatness is in its exploration of many important themes. You can follow the link for a full listing of them, but some of the more important ones are the nature of heroism and of great men, honor, and love.
The Iliad is important in that it is seen as the beginning of western literature -- a story whose influence can still be seen in our literature and language down to the present.
The Illiad tells about the story of the infamous Trojan Horse war.
Its significance-it has impacted many writers and works of literature and is possibly responsible for the proliferation of Greek literature-up till today.
Phrases such as "Achilles heel" have also came out as a result of the Illiad.
It can be considered to be a classic, one that explores the greatness of that war.