mGm I'm not the most experienced having only had mine six months but I have a little knowledge so here goes.
If it is the knob below the bobbin then yes it is to tighten the drive band which goes around the wheel then the bobbin. Sometimes ( especially when first used ) it stretches. The knob simply lifts the bobbin bar a bit further away from the wheel, thus tightening the band. If it feels too tight when spinning then you can loosen it to lower the bobbin.
If it is the knob pointing out towards the wheel then this is to tighten or loosen the band which loops around your bobbin then attaches to the hook next to it, to adjust the tension and get a good spinning bobbin.
Somebody with a more technical explanation will hopefully be along for your query. Until then I hope this makes sense