The fist pump is an integral part of tennis. Everyone loves to fist pump. Some players have it down to an art. It’s a way of showing that you’re fighting. It’s a way of letting your opponent know that you’re in it to win it. It’s a way to show that you’re still in the match. It’s also a way to intimidate your opponents.
What a fist pump is NOT is a way to say “Yes! He/She made an error! I won the point!” That’s just not good sportsmanship.