Definition of twice
Twice means "twice". When you hear the phrase "twice" at a poker table, chances are it is with the phrase "run it twice". When you "run it twice", you are drawing 2 flops, 2 turnovers and 2 rivers. The situation occurs in an all in game. For example, in a live casino, you are all in preflop with AK. Your opponent has JJ. You know it's about a flip (a 50%-50% situation). To reduce the variance, you ask him if he wants to run it twice. If he accepts, 2 full tables will be drawn and you will play for 50% of the pot size each time. It is also possible to run it twice from the flop and turn if all players are all in.
What is the purpose of run it twice?
The purpose of run it twice is simply to reduce the variance. The more hands you play, the less variance will impact you. Running it twice is like playing 2 hands in 1. If you are not paying extra rake to run it twice, it is highly suggested to run it twice. Plus, it's more exciting than a regular hand.