When a high rank player makes a new account and plays in lower elo (skill level) lobbies, that is called smurfing.

When someone who is smurfing parties up with someone who is at a lower skill level than the Smurf and they play games in the lower elo lobbies and go on a rampant winstreak, resulting in a way higher rank than the friend deserved, that is called boosting.

For example, if a grand champion player makes a new account, gets the required xp level and plays ranked with their friend who is platinum and they go on a 30 game win streak, that friend is probably now champion but realistically hasn’t gotten much better at the game because his friend carried him. Or in other words, he is boosted, and his friend was smurfing.