Forking is when you take an existing circuit and make a copy to work on. The changes you make to the new circuit do not affect the original, forked, circuit.
This is a great way to get started, use something that's already been built!
Here's how: Goto the detail page (overview) of any circuit or component and choose Fork from the upper right side. Doing this will copy and add the circuit or component to your own dashboard.
Update: You can not add non-breadboard components to a breadboard circuit, even if you've forked them.