Definition of weak
When we say that a player is weak or that a player seems to hold a weak hand, it means that the player is weak and the hand is weak. A weak player is a player who will not bluff enough, who will not be aggressive enough and who will be easily intimidated (i.e. easily bluffed). A weak hand can be guessed, for example, when a player has only called preflop. If a player raises in the small blind position and the player in the big blind position only calls the bet, we assume that his hand is weak (otherwise he would have raised).
Don't be a weak player
If you are too passive and get bluffed easily, you will be called a weak player. A weak player gets bluffed a lot. The weak player, even if he sees a good situation to bluff, will not take the opportunity. The weak player will be the one who has an aggression factor of 1 or less if you look in your HUD with PokerTracker. The weak player will also be the one who doesn't go to the showdown much (less than 25% of the time). The weak player is usually a loser.