Big transmission problem, or not a big deal?

I bought a car maybe about 9 months ago and havent had any problems with it until recently. I brought my car in to get it inspected about 2 weeks ago and was told i didnt need anything done. not even a week after that my car would jerk sometimes when i would try and reverse. i brought my car in and they said i was running low on transmission fluid and tightened a hose so that it wouldnt leak and told me it will be fine. about 2 days after that i drove maybe 20ft from my house when my car started making really weird noises. i pulled over and put the car in park and it started to roll. after driving the car a little further the noises stopped and i brought it back to the mechanic. he didnt seem to have a problem with my car rolling but said that it could be a problem with it not locking when put in to park because something on the tranny was worn away. i still dont know what the noises are from. could it just be a minor issue, or do you think i'll need to replace my transmission soon? I dont know anything about cars, so if you could please explain it without car terms that only a mechanic or someone that knows a little bit about how cars run would know, i would greatly appreciate it.

Sarah87....Alway state the year of the car, engine size and mileage. Miles you drive per day. Highway or Stop and Go ???

JzJzz