The most common situation in poker, before all the cards have been dealt, is that you don't have the best hand yet but can have the best hand if the right card comes up. That card is called an "out". If you have a makeable hand you will have more than one out. It is especially important to count outs when you must call or fold. But even if everyone checks, it's good poker technique to always count outs. It becomes second nature and you don't have to remember to do it when you really need it.
Some players never count outs. They say, "How can I know what the other players have in the hole?" The point these players are missing is that that counting outs is not an exact science. But you can get a reasonable picture of your chances to win the hand by counting outs. The better players count outs and then use the information to calculate the pot odds.