The process of shuffling an opening hand back into the library and drawing a new one. Under the London Mulligan rule, a player draws a fresh 7 each time but puts one additional card on the bottom of the library per mulligan taken. In multiplayer games and Brawl, the first mulligan does not count toward the bottom-card count.
For new players
A hand with no lands or only lands is almost always worth mulliganing away.
You have an opening hand that does not fit your plan, so you choose to mulligan. Shuffle it back into your library, draw a new hand equal to your starting hand size, then put cards on the bottom equal to the number of times you have mulliganed. Once you keep, you cannot mulligan again.