The phrase "simple knob" refers to a basic or uncomplicated knob, which is a small object that you can turn or push to control something. It implies that the knob doesn't have any fancy or complex features. Full definition
There's a lovely simple knob for operating in sequential - manual mode. (automobilemag.com)
The center stack houses a trio of simple knobs for HVAC, a series of toggle switches, and a multifunction radio — the latter being too small to integrate a navigation system. (automobilemag.com)
Children usually start out with simple knobbed puzzles that are outlines of simple shapes that fit into corresponding board cutouts. (teach.com)