Definition of upswing
In poker, when we talk about an upswing, we are talking about when you are on a winning streak. Let's take a player who makes, over the long run, 5bb/100 hands. This is his hourly rate. Over the long term, he will consistently win 5 big blinds per 100 hands. On the other hand, in the short term, there are months when he will make 1 big blind per 100 hands, other months when he will lose 2 big blinds per 100 hands and months when he will make 10 big blinds per 100 hands. This is called variance. The fluctuations are called swings. Some swings are downswing and some swings are upswing.
Don't be distracted by the swings
This is easier said than done, but whether your results are good or bad, you need to keep the right attitude. During downswings, some players will become very fearful. During upswings, some players will be far too aggressive and will tend to bluff more than is optimal. Try to always play the same poker; don't stray from your winning style, both in downswing and upswing.