What is boosting?
Boosting is when a player has more than one account and ranks on both. They then surrender one account to the other over ranked to 'boost' the other account up on elo. Then they continue to rank on their other account to regain the lost elo and repeat the boosting process. This inflates elo far past the user's actual skill level and makes leaderboard positions not indicative of skill and is highly frowned upon within the community, with some players creating low elo accounts specifically to 'snipe' and get matched with the booster in ranked to drag their elo back down again.
Boosting is often self defeating because once an account is artificially much higher in elo than it would be playing fairly the user will not be able to sustain that elo by playing normally with the account, and thus will have to continue to boost forever to maintain the elo - but boosting only works when you can repeatedly match your two accounts together. Because it requires continual boosting to keep the elo on the boosted account - due to elo decay from inactivity - and it is a bannable action in the game, some boosters may find themselves IP banned by the creators and they will not have any membership they had refunded. However, some consistent offenders create new accounts using a dynamic IP address to boost once again.
For the most up to date and actual evidence on boosters go to the general section of the forums where players often 'report' boosters and can then speak out against fake accusations as well.