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"))