[UNSOLVED] How to Double Key Down Event?

I've been having troubles with the so called quick double w press for run on minecraft stuff AKA Double key down on "w" and when I typed the ^Title^ name down in questions here I got no results :/

So far this is an example I've found in Roblox Forum>Scripters but can anyone please describe it to me what each line does and/or make a better understandable version? It'd be highly appreciated to give <both< and show a full script not like a incomplete one down v here. v

if key == "w" then
if tick()-last < 0.25 then
--Sprint code
end
last = tick()
end

Thank you for looking.