Poseidon was the god of the ocean
The Greeks thought of Poseidon as a god of violent, unpredictable movement. He is most often the god of the ocean, which is of course the biggest, most unpredictable, and most dangerous thing around. Many Greeks spent a lot of time sailing on the ocean, and they paid a lot of attention to Poseidon.
There are not very many stories involving Poseidon. One is the story of Phaedra, which shows Poseidon more or less as a blind, uncaring force, rather like the oceans and earthquakes he controls. Another is the Odyssey. You might think of Poseidon as representing physis, nature, in the Greek mind, and that when people sacrifice to Poseidon they are trying to control nature, to reduce chaos to rationality, to establish nomos, or law. For people who sailed on the ocean in tiny wooden ships, the idea of being able to control the ocean might seem very interesting!
To find out more about Poseidon, check out these books from Amazon or from your local library:
Poseidon, by B. A. Hoena (2003). For kids.
D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire.
Greek Religion, by Walter Burkert (reprinted 1987). By a leading expert, for adults. He has sections on each of the Greek gods, and discusses their deeper meanings, and their function in Greek society.