A bad call is a call which has a negative expected value. That is, the player, if he continually makes such calls, will lose money in the long-run.

Obviously players tend not to make intentionally bad calls. A player however does not know exactly what the other player has and cannot predict for certain what they do on further streets. Thus any call has a level of uncertainty. Predicting the value of the call can be done by calculating the implied pot odds.