In poker, turning a hand face up refers to taking an action with a certain hand which makes it obvious what your likely holdings are. For example, a 3-bet pre-flop by a tight/passive player is very likely to be with AA, KK, and sometimes with QQ.
The key to being able to put someone on a hand is to know your opponent’s playing style and tendencies, so that you can start to more accurately assign hand ranges in any given situation.
At the low stakes, you will regularly come up against weaker opponents who are predictable because they play their hands face-up. In certain spots, some players will have such a narrow range that you can put someone on their exact hand or very few possible likely holdings, and that makes them very easy to play against.