UrW tries to simulate your character's limited knowledge about plants by giving randomly assigned "don't know" names to them. You can try to increase the herb lore by frequently trying to identify them, which gradually increases the skill which eventually allows you to identify them all. Once my starting character has enough food to relax a bit I try to perform a weather lore and identify each of a number of carried leaves each morning when awakening, evening when going to bed, before eating, and when waiting for fatigue to wear off before doing something quality related (such as tanning hides). Note that you can't spam identification attempts, so some time will have to pass between each attempt.
The general advice is not to eat unidentified things, in particular mushrooms.