A grosso modo a "poker out" is any card that could help improve your hand. Before starting to play you should learn the classification of hands, that is, which cards have the most value and how to play them to win. The more time you spend practicing and counting poker outs, the easier it will be.
In poker, an "out" refers to any card that can improve a player's hand to a winning one. For example, if a player holds two suited cards, and the flop contains two more cards of the same suit, that player has a flush draw and has nine outs (the remaining cards of that suit in the deck) to make a flush on the turn or river.
The odds of hitting an out can be calculated using the "Rule of 2 and 4" for a quick estimation. This rule states that after the flop, you can multiply your number of outs by 2 to get a rough percentage of hitting the card on the turn, or multiply by 4 to get the rough percentage of hitting the card by the river.
For example, if a player has a flush draw with 9 outs, they have about a 36% chance of hitting their flush by the turn (9 x 4 = 36), or a 18% chance of hitting it by the river (9 x 2 = 18). However, it's important to note that these odds are only estimates and should not be relied on completely, as there are many factors that can affect the final outcome of a poker hand.