Towerkill is a term used in video games, specifically in games like League of Legends or Dota. In these games, teams of players compete against each other, trying to destroy each other's base.
Each team has a base with some structures called towers that help defend it. Towerkill is when one team destroys all the towers in the other team's base. Once a team's towers are all destroyed, the other team can easily destroy the base and win the game.
Imagine you and your friends are playing with building blocks and you each have a tower made of blocks. Your friends want to knock down your tower. To do this, they have to first knock over the blocks at the bottom of your tower. Once all the blocks at the bottom are gone, your whole tower will fall down.
In games like League of Legends or Dota, the towers work similarly. The teams have to work together to attack and destroy the towers, one by one, until finally, they can make their way into the other team's base and win the game.