Definition of face up
Expression face up has several meanings. One of these meanings is when we talk about cards that are "face up" (face up). When the dealer deals flop, he puts 3 community cards "face up", that is face up, visible to all. We can also use the expression face up to refer to a player who plays as if we can see his cards. A player who plays face up is an easy to read player, who plays without refinement, as if you could see his cards. These are often players who are not well balanced.
Don't play face up!
To avoid playing face up, you have to balance your range. The classic example of a face up player is a silent player who lays down hand after hand and suddenly raises to preflop big blinds preflop, suddenly looking interested in the game. Playing face up can also be the result of not being balanced. If you never bluff on the river when you're all in, your opponents will probably notice and stop paying you when you push all in into the big pots on the river.