I think I answered this question on a different category for you haha, so I will just copy and paste the answer.
There are a couple questions one needs to ask before getting a positive answer, because you didn't really clarify.
1. Does your vehicle move at all? Or does it just rev up when you put it in drive and the vehicle doesn't move?
This could possibly be a misaligned shift linkage, and its not putting it into drive fully.
Or some of the syncronizers in your transmission are failing, and not being able to put power to the wheels because the clutches aren't activated and holding when they are supposed to (assuming its an automatic)
2. (this is for manual transmissions) If you can get the vehicle to move, but putting your foot down to accelerate makes the RPM's climb and you aren't accelerating at all, then it could be a bad clutch. To test this on your own, go down the road (about 30-40 miles per hour) put it in 4-5 gear, press in the clutch, rev it to about 3-4000 rpm and then let the clutch out, while keeping your foot on the gas if the RPM's don't immediately fall down to where they are supposed to be and kind of just gradually goes down then it could be a bad clutch.
ALSO depending upon how bad it is, get the vehicle to about 30-40 mph, put it in 4th gear and just put the gas pedal to the floor, if the RPMs climb but you are not accelerating, you have a bad clutch and it needs to be replaced.
Hope this helps!!