A knob was a rounded handle used to operate a device.
In 2375, Tom Paris defended the Captain Proton design elements he had used in the creation of the Delta Flyer, saying "every one of these knobs and levers is fully functional." Tuvok countered that they were unnecessary, to which Paris explained that he wanted to feel the starship he was piloting, rather than touching a screen. (VOY: "Extreme Risk")