What does the Terminal Application do?

Terminal User Guide for Mac - Apple Support

To reinforce macjack’s admonition, do not even open the Terminal app unless you are skilled in the arcane language of Unix, the underlying engine of macOS. Issuing commands in the Terminal app is a sure way to really destroy your macOS installation unless you know exactly, precisely what you are doing.

Terminal is way to communicate with the Mac at the system level, without using the GUI interface. If you don't know what it is it's best to stay away from it. The wrong command could bring your Mac to it's knees. If you do want to learn there are many tutorials.