Definition of push
A "push" is literally a "shoot". What is meant by a "push" is when a player takes his entire chip stack and pushes it to the middle of the table. Therefore, there are all in "push" that are usually associated with tournament poker. In tournament poker, when a player runs out of chips, he is in a "push or fold" position, meaning that the only viable options for him are to push his entire stack or fold. In other words, there is no more room to maneuver as there was when he had a good stack. Normally, we can raise 3BB, flop and fold if our hand is too weak and we have resistance, but when we have 6 big blinds for example, our options are limited.
Push or fold according to the Nash card
In tournaments, when we have 3bet big blinds and under, it's time to eliminate certain options. Normally, we can fold, raise or 3bet or 4bet, etc. but with so few big blinds, only two options are to be considered: push or fold. To do this, you can use a chart based on the balance of Nash, the great mathematician. This chart will give you the mathematically profitable range of hands to push depending on your position, hand and number of chips.