What fork do you have? The Trek website is not very clear on this. In the specs they show a Spinner 300 and the photo is of an RST.
Trek Bikes | Bikes | Mountain Hardtail | 4300
Anyway, a suspension fork can have as many as five different adjustments or settings:
The way these adjustments are done depends on the type of fork. Some low end forks have no adjustments and some more expensive forks have all of the adjustments listed above. Also, there are several different ways that a fork provides damping. Some use springs, some air, and others elastomers.
Here's the short answer for each setting:
Compression is the speed at which the fork reacts to bumps or irregularities in the terrain and moves from it's fully extended position to fully compressed position.
Rebound adjusts the speed at which the fork returns to it's extended position.
Preload adjusts sag. When you sit on th bike, a portion of the stanchions (the rods that extend down from the crown) will compress into the lowers. The preload adjustment will increase or decrease that distance. Check your owner's manual on this one because the sag setting will vary. If you don't have a manual, start around 25%. So if you have a 100mm fork, set preload so 75mm of your travel is available.
Lock out Grape already covered.
Travel is just that. How far the fork is allowed to travel up and down. Some forks are adjustable from say 100mm to 120mm to 140mm for instance. Others are not adjustable at all.
Hope this helps.