The leader key could be used as a way to create a mapping with a key-binding that can be overridden by the end user.
The leader is the \ key by default. In order to override it, the end-user would have to execute
:let g:mapleader='somekey(s)' before defining the mapping.
In a typical scenario, the mapleader is set in the
.vimrc, and plugins use
<Leader> in the keybinding part of their mappings to have them customizable.
In the plugin, we would define mappings with:
:nnoremap <Leader>a somecomplexaction
This would map the somecomplexaction action to the \+a key combination.
The a action without the leader does not change.
It's also possible to use
<Plug>Mappings to leave more room to customise plugins keybindings.