What does a % do in a script?

So I was looking at the Snack Break problems created by Sukinahito. I looked at his solution for Snack Break #1, and his answer included % marks which I didn't even know what they did in Lua.

What do % marks even do in ROBLOX Lua?

Sukinahito's script in question:

local results = { }
for index = 1, 100 do
	if index == 0 then
		results[index] = "FizzBuzz"
	elseif index%3 == 0 then
		results[index] = "Fizz"
	elseif index%5 == 0 then
		results[index] = "Buzz"
	else
		results[index] = index
	end
end
print(table.concat(results, "\n"))