Can someone please describe how this for loop works? concise title o.O

A for loop is apart of a family known as control-flow; you can denote a form iteration built from a condition to define how the code will iterate through the instructions n it's scope. Alternatively, we can use iterator functions to define specific flows for specific scenarios.

A common iterator function is the enumeration function. This will allow us to traverse through a list of elements in a form of key-value pairs, meaning we can attain an index—where the element in rotation is in the given table—and a value—what that element is.

The Index and Value are usually represented as i and v, but can be named anything we want.

local arbitraryTable = {4,6,1,4,5}

for i,v in pairs(arbitraryTable) do
	print(i, v)
end 

--[[ Output:
Where in the table:		Value of that element:
	    1						4
	    2						6
	    3						1
	    4						4
	    5						5
]]

Regular For loops

Let's talk about the most common use of a for loop. As aforementioned above, these loops can define how we loop (traverse or iterate) over a scope of instructions. A normal for initialization follows a format that can be visualized like so: start stop step (optional). By evaluating these numbers, they will determine how the loop shall act. Let's take a look at a few examples:

for i = 1,10 do
	print(i) 
end

This tells the for loop to iterate over this print() call from a range of 1 to 10 (up to 10 times). i represents the current iteration we're on in respect to the range given:

--[[ Output:
	1
	2
	3
	4
	5
	6
	7
	8
	9
	10
]]

Let's take a look at an example with step, this defines how we step through the numbers from 1-10, default, it's an increment of 1, but let's say we want to step to every second number:

for i = 0,10,2 do
	print(i)
end

--[[ Output:
	0
	2
	4
	6
	8
	10
]]

for i = 10,0,-1 do --// From 10, to 0, step back 1
	print(i)
end

--[[ Output:
	10
	9
	8
	7
	6
	5
	4
	3
	2
	1
	0
]]

TL;DR: It goes through tables and allows us to perform actions on each individual attribute. In the example you have, it runs through each player of the game.