You've probably already encountered a key referred to as the leader key. The leader key is essentially a namespace for user or plugin defined shortcuts. Within a second of pressing the leader key, any key that's pressed will be in from that namespace.
The default leader key is a backslash \, but it's not the most comfortable binding. There are a few alternative keys that are popular in the community, with the comma (,) being the most popular. To rebind the leader key, set the following in your .vimrc file:
" Map the leader key to a comma.
let mapleader = ','
You'll want to define your leader key closer to the top of .vimrc, as the newly defined leader key will only apply to mappings defined after its definition.
When you rebind a key, it's default functionality is overwritten. For example, comma , this is used to replay the...