A "hypocrite" is someone who loudly pretends to be good, but secretly does bad things. Or someone who does the same bad things that they criticize others for.
For example, a preacher who teaches his congregation not to chase after money, but actually lives in an expensive house and drives a luxury car himself, is a hypocrite.
Another example is a parent who tells her teenage daughter not to smoke, but is secretly a smoker herself.
Politicians, religious leaders, parents, and teachers are often accused of being hypocrites.