A nub is a little piece of something, especially one that sticks out. The raised rubber buttons on your TV remote control are nubs.
Tiny lumps or bits are nubs, like the nubs on tree branches that will become buds in the spring, or the nubs on a crocheted blanket, little woven bumps. Another kind of nub is the heart or essence of something: one incriminating piece of evidence might be the nub of a judge's decision in a case. The "lump" meaning is older, stemming from knob, while the "essence" meaning dates from the nineteenth century.