The proof builder lets you interactively build proofs using all of the publicly-available theorems and rules of inference. It can make working with proofs easier, apply rules of inference correctly, show what inference rules do and how they work.
Note that the proof builder tool needs documentation that does not yet exist. Today using it is still an adventure. You might like to start with the version tuned for solving simple math problems at the Mathtoys website.
Note: Proof displays and the Proof Builder are supported in recent versions of Firefox, Chrome, and Microsoft Edge.