Gigantic is an adjective used to describe something that's really big, as though it were made for a giant. You might call a skyscraper a gigantic building, especially if it towers over other buildings nearby.
As you might imagine, gigantic is derived from the noun giant. Giants were mythical beings that were said to be of immense size and strength. Literally, gigantic means “of or relating to a giant,” but in practice gigantic is widely used to describe almost anything as especially big. If an article of clothing is way too big for you, you could call it gigantic. You might also complain of having a gigantic headache because someone is being a gigantic pain.