A swing is a large shift in the size of your bankroll or how you're currently doing in a game. If it's positive, it's an upswing, if it's negative, it's a downswing. Swings aren't just dependant on play, rather they are the result of the random components of poker, which lead to lucky cards or unlucky ones for a short period of time.
Bankroll swings can be any size, depending on the type of opponent and your own playing style. Phases can occure even if you're playing solidly, if you loose several buy-ins in a row. Conversely, there may also be phases where you win a disproportionally large amount of money, as you brifely get lucky cards to your benefit.
In the long-term, up and downswings will balance out and the only remaining losses will be due to incorrect play. It is important not to deviate from correct play, or even go on tilt, during these phases.