What is the purpose of this in for loops?

I've seen this being used many times, but never really understood it.

Why is it that some people write their for loops like this:

for _,v in pairs(table) do
    --code here
end

Instead of this?

for i,v in pairs(table) do
    --code here
end

If you didn't see the difference, it's that instead of an "i" (or some other argument) being there, they put a "_". Is this just a name for the argument? Just a style some people do? Or does it *actually* do something?