Material that it is in the public domain is free from restictions under copyright law. 
This may be becuase the intellectual property rights have expired, been forfeited, or are inapplicable. Good examples of public domain content are the works of Shakespeare and Beethoven, most of the early silent films, the formulae of Newtonian physics, and the patents on powered flight.

But, as with many things, it's not always that simple. Depending on where you are in the world, the process or criteria by which a piece of work enters into the public domain might be different.

A good starting point for understanding the public domain is to read Wikipedia's article on the public domain. Once you're familiar with the concept, a good follow-up is the Creative Commons wiki article on the public domain.

Now that you've got a grasp of what the public domain is, it's time to start content hunting!