the shell

Unlike past Linux, modern Linux almost always doesn't require the use of terminal but if you do use the terminal then you'll be working in a shell. Bash is the first of these and is on every distro, there will be times where you'll have to use it no matter what. The other two are ZSH and Fish. Fish is aimed at beginners but lacks some functionality while ZSH can do basically everything Fish can but requires setting up. Both can be modded though and are very powerful once you've gotten used to learning Bash (think of this as learning C before picking up C++ and Rust).