Definition of weak
When we say that a player is weak or that a player appears to hold a weak hand, it means that the player is weak and the hand is weak. A player who is called weak is a player who will not bluff enough, will not be aggressive enough, and will be easily intimidated (here we mean that it will be easy to bluff him). A weak hand can be guessed, for example, when a player has only called preflop. If a player raises to position small blind and the player in the big blind position just calls the bet, we will assume that his hand is weak (otherwise he would have over-raised).
Don't be a weak player
If you are too passive and bluff easily, you will be called Player weak. A Player weak gets bluffed a lot. Player weak, even if he sees a good situation to bluff, will not take the opportunity to bluff. Player weak will be the one who will have an aggression factor of 1 and less if you look in your HUD with poker Tracker. Player weak will also be the one who will go little to the slaughter (less than 25% of the time). Player weak usually loses.