Asphalt is a liquid commonly found in tarry biomes. If you step in it you will probably get stuck in it for a while and receive a temporary DV penalty. Thus, it is desirable to avoid stepping in it if you are in a dangerous area and annoying otherwise. Asphalt can also burn without destroying itself so heat based weapons are not advised in areas with lots of asphalt.
Asphalt is sometimes an ingredient for the cures of fungal infections and Glotrot
It is worth nothing at all, so unless you are sure you need it for something, you shouldn't carry it with you.