What is a priori knowledge?

A priori knowledge is knowledge that can be acquired without having to 'look at the world'. A priori knowledge is independent of the empirical world in two ways: i) it's truth does not depend on anything happening in the world and ii) nothing that could happen in the world would make it the case that an a priori tryuth stopped being true. The following sentences are all examples of a priori knowledge: 

'All bachelors are unmarried' 

'Two plus two equals four' 

'No one is taller than the tallest person' 

Much philosophical knowledge is a priori because the focus is on the connection between concepts and ideas and not on how the world works. This is why doing good philosophy (unlike doing good science) doesn't depend on going out and finding data. All we need to determine whether something is true 'a priori' is our own power of thought.