Accelerate is an emacs module that lets you change the behaviour of a command when the key invoking it is auto-repeated.
For example, when moving the cursor (with e.g the arrow keys or C-n, C-p, etc) and the key is held down and auto-repeated, then the cursor can be moved in increasingly larger steps.
Any command that takes a numeric argument as its first arg (specified by e.g. (interactive “p”)) can be accelerated. But it is mostly only useful to accelerate cursor movement commands that normally moves cursor a short distance.