ELO is just a number that shows how good you are at a game. Everyone starts at 1000 because the rankings really don’t know who is good yet. When you win, your number goes up. When you lose, your number goes down. If you beat someone who is “supposed” to be better (higher ELO), you gain more points. If you lose to someone who is “supposed” to be worse, you lose more points.
At the start of this, everyone is at 1000, so the ranking thinks all players are equal. Because of that, every win or loss feels like a big surprise, and the ranking makes big changes to people’s scores. That’s why MotherFart loses a lot of points for getting 4th place. After more games are played, players spread out to their real skill levels, and the score changes get smaller and more stable.
TL;DR - ELO is a skill score. Everyone starts at 1000, so early wins and losses cause big swings. It stabilizes once players spread to their real skill levels.