What is the meaning of see no evil hear no evil speak no evil?

The saying is associated with the "three wise monkeys" where one monkey has his ears covered, one his eyes, and one his mouth. The meaning is different to different people, but probably originally it meant "Don't participate in evil in *any* way."

Some people use it to refer to people who turn a blind eye to the truth, but it doesn't have to mean ignoring evil when it exists. I think it more means that we should not be allowing evil to come into us (through our eyes or ears), or out of us (through our mouths), not that we shouldn't be trying to make our society better and less evil.