When you take the Search action, you devote your attention to finding something. Depending on the nature of your search, the GM might have you make a Wisdom (Perception) check or an Intelligence (Investigation) check.

From experience, most players and DMs interpret this wildly differently, although I can't exactly blame them because in 5th edition the rules for perception, hiding, invisibility, and searching are all split up and located in different parts of the book.