Definition of good run
A "good run" or "run good" is a short or long pass during which you are lucky. When we are in a good run, we have aces and our opponents have kings, we have stronger brelans than our opponents, we have ace colours when our opponents have king colours, in short, we have the momentum, the wind in our sails. The "good run" is opposed to the "bad run", a pass during which our aces are cracked and during which no matter how strong our hand is, we always have the second best hand.
The good run and the bad run are in our heads
Obviously, cards have no memory. If you have 3x aces in 3 hands, it's a good run. But in reality, it is only your mind that notices because you will have aces every 221 hands in the long run. The good run and the bad run only exist in our head, so you shouldn't give it too much importance. On the other hand, it can give you a certain momentum. If you win 4 big pots in 15 minutes, it may discourage some players from playing against you.