Squealer is the smooth talker: the minister of propaganda: the one who spreads the (increasingly corrupt) philosophy of Napoleon's brand of Animalism, which eventually turns from socialism to an oppressive totalitarian state. Squealer is the talker and convinces all the animals of Napoleon's demands. Hence the name, he squeals propaganda. Think of Squealer as Joseph Goebbels, the minister of nazi propaganda. Since Animal Farmis Orwell's mockup of the Russian Revolution, Squealer is often compared to the newspaper Pravda. Squealer functions as this Soviet newspaper did, explaining and justifying tenets of the Revolution, and at times, even rewriting the past.
Squealer's name is misleading in that today "to squeal" on someone means to denounce or reveal something bad about him/her.
Squealer's role in 'Animal Farm' is quite the opposite: as his leader's impressario, he is constantly embellishing Napoleon's public image, pimping him up to look better than he really is. Squealer is quite good at his job in that he does indeed manage to convince most of the animals of Napoleon's natural superiority and leadership ability.