A rat was a type of rodent that was larger than a mouse. They were typically found on Earth as well as a variety of worlds. In laboratories, Humans would test rats response to stimuli by placing them in mazes and giving them a treat if they succeeded in getting out of the trap. The treat was often a piece of cheese.
In early 2365, while the USS Enterprise-D was entrapped in a void, Chief Engineer Geordi La Forge stated out loud, "Like the rat said: Keep the cheese. I just want out of the trap." Later, Doctor Katherine Pulaski would likened the situation of the Enterprise-D to that of a rat in a maze. (TNG: "Where Silence Has Lease")
A year later, Odo sneaked aboard a Kressari freighter and disguised himself as a rat again to investigate allegations by one of Quark's contacts that the Kressari were arming the Bajoran extremist group known as "The Circle". (DS9: "The Circle")
After she learned about the Srivani experiments on her crew, Captain Kathryn Janeway told Alzen that "these lab rats" are fighting back to demonstrate that she won't cooperate with the Srivani. (VOY: "Scientific Method")