How do I make a cooldown for this animation script?

A lovely thing called a Debounce

A debounce basically means "If soAndSo is not this don't continue. But if it is, continue and wait until I am re-enabled." Or as the wiki states, A debounce system is a set of code that keeps a function from running too many times. It comes from the idea of mechanical switch bounce, where a switch bounces when pushed, creating multiple signals.

To fix your code

local debounce = false --Define it, can be named anything

function onKeyPress(inputObject, gameProcessedEvent) 
	if not debounce then --If the debounce is set to "false"
		if inputObject.KeyCode == Enum.KeyCode.M then
			debounce = true --Set it to true,
			local animation = game.Workspace.Slash
			local anim = script.Parent.Humanoid:LoadAnimation(animation)
			anim:Play()
			wait(5) --"Cooldown"
			debounce = false --Set it to false,
		end 
	end
end
game:GetService("UserInputService").InputBegan:connect(onKeyPress)