leader

https://vi.stackexchange.com/questions/836/what-is-leader

Vim is full of various commands, which are assigned to almost all keys on the keyboard.
But this causes a problem: Which commands can we use for our own commands,
without interfering with existing ones? And at this moment, the <Leader> key comes into play.
Think about <Leader>-key like a namespace for any user-defined commands.
You can assign any command to a mapping with a leading <Leader> and you can be fully
confident that your mapping won't break anything.

By default your <leader> is \, backslash. You can check it with:

:echo mapleader

If this gives you an E121: Undefined variable: mapleader,
it means it's set to the default of \. If it gives you something else, then it's that