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.

so in a for loop, you are usally getting a dictionary, the dictionary has numbers for all of its data like this:

local myDictionary = {"hello","world"}

if I wanted to get hello for instance, you could do this

local myDictionary = {"hello","world"}
print(myDictionary[1])

that would print the first thing in the dictionary, so hello

when using a for loop, you could get things by their number, or there actual object(I and player)

the "pairs" part means to get the dictionary in two parts, I and player.

so I and player are variables to use in the for loop

if you don't need I, you could do this by doing "_" so

for _,player in pairs(Players:GetPlayers()) do

end

but in this case you can only use player, not I

Essentially what a for loop does is it can either take index or an index and a variant. An example of both of these are

Index only

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

This would print every number from 1 to 10. You can also use this as a loop to do something 10 times. There is another way to put this though that uses 3 numbers.

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

So this will count down from 10 to 1 because we sent our increment to -1. That is the third value.

Now another way to use it is to use it on a table.

Here are some examples

for i,v in pairs(game.Players:GetPlayers()) do
    print("Index is "..i.." variant's name is "..v.Name)
end

As you can see, this returns an index and an object. The object being a player because Players:GetPlayers() returns a table of every player on the server.

For more info you can go here