Question

what does the judicial branch do?

Answer 1

It judges civil and criminal cases. It is the system of courts.

Answer 2

I should add that the judicial system also issues warrants for arrest and search, appearance as a witness or juror at a trial, and court orders that MUST be obeyed, even by the an official in another branch of government. As an example, President Nixon was ordered by the Supreme Court to turn over the White House tapes during the Watergate investigation.