In gaming culture, smurfing is defined as logging into a secret account seperate from a main account as to allow the user to play a game without being detected by his or her peers.
Generally the purpose of smurfing is to improve oneself without compromising the statistics of a main account. It allows gamers to try new strategies, or simply play without worrying about a "record".
A person in the higher ranks of a game, usually a team based dps game like overwatch or csgo, who purposly makes a new account or purposly drops down to the lower ranks just to destroy the noobies.