Definition of the term "bad run
Poker is a game of variance. In the long run, we'll get the same proportion of AA as we do 22, 55 or 77, but in the short run, it's possible that our card set will be atrocious. Also, it's possible that even when we have AA, we'll lose the hand to a pair of 2s that hit three of a kind on the flop. When you don't have a good starting hand, your straights lose to flushes, your KKs meet AAs, and your brelans get beaten by better brelans, you are said to be on a "bad run".
Other details on the "bad run
The bad run is something that happens to 100% of poker players. If you are lucky, these periods will be short and you will get away with it. If you are not so lucky, the bad run periods can last for months or even years. The amount of play you put in will also have an impact. If you play 1,000 hands a month, your bad run period could be longer than if you play 2,000 hands a day. The important thing is to stay calm during these difficult times.