Can Someone Explain me This Little Piece of Code?

Simply imagine anything that you put in the place of the parameter sound, ends up replacing it in the code.

local MusicOne = game.SoundService.MusicOne
local MusicTwo = game.SoundService.MusicTwo
local MusicThree = game.SoundService.MusicThree
local MusicFour = game.SoundService.MusicFour
local MusicFive = game.SoundService.MusicFive


local function play(sound) -- Here you have defined a function called play, and defined the parameter 'sound' that you will input later

    sound:Play() -- When you enter the name of a sound (e.g. MusicOne), it replaces the parameter, so it's the same as doing MusicOne:Play()

    sound.Ended:Wait() -- Once the song has ended, the function waits

end

while true do -- This is the beginning of an infinite loop. Any code in this loop will be repeated forever

    play(MusicOne) -- You are substituting 'sound' for MusicOne like in the example.

    play(MusicTwo) -- Once MusicOne ends, MusicTwo plays...

    play(MusicThree) -- And so on...

    play(MusicFour)

    play(MusicFive) -- After this song finishes, you start again from MusicOne

end

--These five lines define sounds already set up in the sound service (I think) 
local MusicOne = game.SoundService.MusicOne
local MusicTwo = game.SoundService.MusicTwo
local MusicThree = game.SoundService.MusicThree
local MusicFour = game.SoundService.MusicFour
local MusicFive = game.SoundService.MusicFive

--This function takes in one parameter, the sound (one of the ones above)
local function play(sound)
  sound:Play() --This plays the sound
  sound.Ended:Wait() --This waits until the sound finishes
end

--Loops infinitely 
while true do
  --Runs the function, play, for each sound
  play(MusicOne)
  play(MusicTwo)
  play(MusicThree)
  play(MusicFour)
  play(MusicFive)
end

This is a basic overview of what happens with this script. If you would like more explanation on the sound.Ended:Wait() part I have a video on my YouTube Channel all about events right here