Definition of face up
The expression face up has several meanings. One of these meanings is when it refers to cards that are "face up". When the dealer deals the flop, he puts 3 community cards face up for all to see. The term face up can also be used to refer to a player who plays as if his cards are visible. A player who plays face up is a player who is easy to read, who plays without refinement, as if you could see his cards. These are often players who are not well balanced.
Do not play face up!
To avoid playing face up, you have to swing your range. The classic example of a player playing face up is a silent player who folds hand after hand and suddenly raises to 12 big blinds preflop, looking suddenly interested in the game. Playing face up can also be about not being ratted out. If you never bluff on the river when you are all in, your opponents are likely to notice and stop paying you when you push all in in the big pots on the river.