A bet which, on average, will tend to be profitable when called. This differs from a bluff, which will only be profitable if the opposition folds. This also differs subtly from betting with the likely best hand; when betting the best hand, you expect to win the vast majority of the time.
When you have a marginal hand, you must carefully evaluate the likely callers. If likely callers include a lot of hands that beat you and few hands that will lose to you, there is no value to the bet. If there are plenty of losing hands that will call yours, then you may have value in the bet.
If you have a marginal hand and your opponents are tight players likely to be on draws, you can have no value -- they will either fold their BustedDraw or raise back if they made.