no offense, as this advice is offered to lots of people new to the boards... the search button is your friend, there are lots of threads about these. Refugiums are a broad subject, but I will try and answer your question in a nutshell.
1) you don't need a refugium
2) A refugium is just a separated compartment, whether actually in the tank or outside, that provides "refuge" for something.
One of the main uses that people use it for is to isolate plants that are not desirable in the main tank for use of filtration purposes. They can also be used to harbor fry/breed parents. When setting up a refugium for filtration, you have an overflow (a device that syphons water out of the tank) into the "refuge" where the water passes through lots of plants and then returned (via pump) into the main tank.
Look at the stickies in the plant section if you are wondering the benefits plants have on water quality. There is a lot to be considered if you want to do this effectively and involves a lot of research for the beginner.
Another perk of having a refugium is that an injured fish can be put there without having to set up a hospital tank.