what could it mean if i have a 1990 nissan maxima and it goes up to 6000 rpm’s before changing gear

If this is a manual trans, good chance your clutch is shot. Same for auto, possibly time for a trans rebuild.

It means you should check the transmission fluid per the owners manual for the proper level. And also note the color of the transmission fluid. If it’s brown/black. you probably need a replacement transmission.