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.