A smurf account is another account in a game that is created to play against opponents with a lower skill level for various reasons.
Many games use a ranking or Elo system to rank players according to their skills and thus allow fair encounters. Smurfing describes a tactic where a player creates a new account to escape this mechanism.
There are many reasons for this. Players with a very high level often have difficulties finding opponents. If you want to play at the wrong time, in the worst cases you simply won't find anyone. But good players also smurf when they want to play with friends with a lower rank, or simply don't feel like playing hard CS:GO matches after a long day. But also testing new builds in LOL or trolling for videos can be a reason for smurf accounts.
By the way, the meaning of the word has nothing to do with smurfs. In Warcraft 2, you still had to search for games with your name being shown to potential opponents. But since "Frazier" and "Boyko" were too good, opponents were jumping off them en masse and they simply couldn't play properly anymore. For this reason, they decided to call themselves "PapaSmurf" and "Smurfette" and thus lured new opponents into matches, which they subsequently played into the ground.
Many gamers and game developers view Smurf accounts very critically. Many games take into account the level of the opponents you played against in their reward and match making systems. Losing to players of the same rank usually has much greater consequences than losing to semi-pros. However, if they disguise themselves as "Smurf4000" and make life difficult for you, you won't really progress and won't have much fun in the game.
Still, in many games it is a gray area. Some games take rigorous action against smurf accounts (for example, Fortnite), while others are less critical (for example, LOL).
Due to the large reach of Twitch streamers, "smurf", "smurfer" and "smurfing" have also established themselves outside the gaming community as terms that describe when a professional disguises himself as an amateur in order to make a funny video out of it, for example.