A mod is a player like us, hand-picked by the devs(creators of this game) to be a moderator of the game, usually shortened to just "mod".
Mods have the power to "silence" other players if they break the Terms of Use of this game. Some examples of breaking the ToU are bypassing, cybersolicting, sexting, spamming, homophobic behavior, racism, and more.
They also have the power to "lock threads". When a thread is locked, no one can post on it. A thread is usually locked when it goes off-topic, an argument occurs inside the thread, and if the thread is against the ToU.
The mods are usually hated on, normally by players whom they have silenced. I suggest if you've been silenced before just move on and accept it, whining about it just makes you look like a baby.
And, don't sext. Look at Royale's "The Scary Truth" thread, if it doesn't stop you from sexting, nothing will.