A smurf is a player who actually has a high skill level in a particular game, but has a lower in-game level. In other words, it is a player who has the in-game level of a beginner or slightly more, but actually has played tons of hours and is highly skilled in the game. Smurfs do this by using a different account than their actual account. On their actual account, they may have a high in-game level.
If you have been playing competitive online video games for a while now, you may have come across a smurf yourselves. For example, if you play CSGO and you are currently at the Gold Nova 4 rank, you expect your teammates and enemies to have a similar rank in competitive matchmaking. But if you come across a player that is playing way better than how a Gold Nova 4 player should play, then they are most probably a smurf.
A player doesn’t need to have a really high skill level to be considered as a smurf. A player can be above average or just average and be considered as a smurf. In the above example, if the player is only Legendary Eagle in their primary account, they are still considered a smurf because they won’t have a tough time winning against Gold Nova 4s. In the same way, if you start playing on the bronze rank with another account, you will also be a smurf.