Smurfing is knowingly and deliberately playing at the incorrect skill-level/mmr/elo. Most times, people will create or buy a separate account that somehow has a lower mmr/elo attached to it, to do this. You can ‘play bad’ or ‘throw the game’ for 50 games in a row and your rating will tank a lot, so that’s a bummer to encounter as your new teammate randomly… Then this player can win a lot of games in a row playing as ‘themselves’ and completely stomp games all the way back to their ‘true’ mmr. Does that help explain it?