I'm assuming that you are trying to describe a generic for loop here.

Generic For Loops

Generic for loops are one of two types of for loops in lua. the first being numeric for loops, which are formatted like such:

for iterator = start,finish,increment do
    ...
end

Well, Generic for loops are formatted like:

for key,value in pairs(table) do
    ...
end

Generic for loops can also be formatted like:

for substring in String:gmatch(pattern) do
    ...
end

There are three iterators that can be used when constructing a generic for loop, pairs, ipairs, and next. Ipairs being best for arrays, and pairs and next being best for dictionaries.

Each time an iterator function is called, it will return the "next" value from a collection

References

iterators

generic for loops

generic for loops(2)

Hopefully this Helped!

Extension to theking48989986's answer

His answer is explained very well but I assume you want to know what the _ means.

The underscore (_) character isn't anything special in Lua; people use it as a placeholder. Say you had a function that returned two values, but in the situation you're in, only the second return value is necessary.

local function fn()
    return {}, 8
end

local tbl, num = fn()

You can't just skip tbl and put local num or else num is a reference to the table. It doesn't hurt to still have your tbl variable but you don't need it. So people use an underscore to show they don't care about a value.

local _, num = fn()
print(_) -- it still works as a variable! (not encouraging you do this for important ones)

So people use _ in generic for loops to show they don't care about the index but are still required to fill it in so the codes are valid.

for _, v in pairs(tbl) do
    ...
end

-- No difference! 

for i, v in pairs(tbl) do
    ...
end

It's just visual preference.