Trophies is a representation of player's strength in PVP and give other players an indication of chance of winning an attack.

You obtain trophies if you win an attack, a revenge you initiated or a defence battle.

Trophies can be lost in the following ways:

If a player challenges you and wins the battle, they will steal a certain amount of your Trophies (that includes revenge initiated by another player).
If you challenge a player and lose the battle, you will lose a certain amount of Trophies.

In top leagues, Trophies will be reset at the beginning of each season so that everybody in the league will start with the same amount.