Generally-
A Fender Jazz has...
One knob = NECK pickup volume
One knob = BRIDGE pickup volume
One knob = "TONE"(Fender "Tone" pot adds some treble/brightness).
You can pluck the strings ANYWHERE! That's one of the perks of playing!

You should(or you will) notice-
The strings
feel & vibrate differently when played up by the neck vs. when played back by the bridge vs. when played directly between the 2 pickups...the result is a different 'tone'. How much or how little 'force' you exert will also factor in.
Good luck!