Odysseus learns many important lessons while sailing home to Ithaca from Troy. The most significant lesson teaches Odysseus the dangers of hubris, or excessive pride in oneself.
Odysseus experiences great suffering while he is sailing home. Poseidon, the god of the sea, has a grudge against Odysseus, and because Poseidon is the god of the sea, he can and does make things very difficult for Odysseus as he sails home.
Poseidon's grudge against Odysseus originates from a very specific incident: while sailing home from Troy, Odysseus encounters a cyclops. He manages to escape from the cyclops using his intellect and wit, saving a number of men as well as himself and blinding the cyclops in the process. While sailing away, Odysseus can't seem to resist the urge to brag, so he identifies himself to the cyclops, using his real name and showing off his cleverness now that he is safe. This move turns out to be the antithesis of clever, as the cyclops is the son of Poseidon, and Poseidon resolves to avenge his son.
So even though Odysseus is renowned for his intelligence and his ability to strategize against his enemies, at times he causes problems for himself when he feels too proud.