These are the circular platters seen on CDJs as well as many DJ controllers. Jog wheels replicate the look and function of traditional vinyl turntables, allowing DJs to scroll through records, scratch and ‘nudge’ the timing of a track back or forwards slightly.
If you plan on traditional beat-matching – ie. syncing up the timing of two records by ear – or hip-hop style turntablism and record scratching, then jog wheels are a must.
They’re not essential if you’re happy to rely on the automatic beat-matching capabilities of your chosen DJ software though, which is certainly a viable option given the power of modern applications.