In most areas of poker, weak players are either far too timid or far too aggressive. In both cases, their value betting tendencies become extremely skewed - particularly in terms of calling. Timid players generally fold too much, but they actually call too much late on in hands. If they get to the river they will generally not be going anywhere if they have any type of hand, generally because they have decided that they need to make a stand. This makes their range typically quite weak and their chance of calling quite high - a perfect combination for thin value bets.
The over aggressive player type also generally calls too much on the river, simply because they apply their own thought processes to those around them. As they make ill-timed bluffs constantly, they expect everyone else to do the same, and see any hand with showdown value as a potential calling hand on the river. Again, a great combination for exploiting with thin value bets.
These two player types also both value bet very badly. The timid player just wants to get to showdown, so you will rarely be up against marginal hands when they bet late on in hands - making marginal hands relatively easy folds in most cases. The overly aggressive player also often wants to get to showdown with any kind of marginal hand, choosing to bet only with big hands or hands that have no showdown value, meaning that you rarely encounter these players trying to get maximum value out of their marginal hands.
This combination of calling value bets far too much, as well as not making their own in the right spots, is a lethal combination over time. Extracting maximum value from the marginal hands of poker is probably the single biggest divide between weak and good players, and understanding when and why to value bet can very quickly seperate you from the field at nearly any table.