Swing is a larger movement in the amount of your own bankroll or current performance in the game, either in a positive way, which is called Upswing, or negatively, what is called Downswing. Swings are not only game-related, but simply a result of the random component in poker, which means that in the short term the card luck is for you or against you.
Depending on the type of opponents and their own style of play, swings in the bankroll can be larger or smaller. Even with proper play, you may experience periods of losing multiple buy-ins in a row. On the other hand, phases can occur again in which one wins excessively, because the short-term luck of the card turns out in its own favor.
In the long run, up- and downswings will balance each other out and the remaining losses are the result of faulty play. It is important not to be distracted from the correct style of play and strategy even by longer lasting swing in one direction or even to go on tilt.