When it comes to backpacking gear, one of the most important pieces is the footprint or groundsheet. This piece of gear protects the bottom of your tent from abrasion and wear, and also helps to keep moisture and insects out. While a tarp can technically be used in place of a footprint, it's not really ideal for a few reasons.
First, tarps are usually made from much heavier-duty material than footprints, so they tend to be significantly bulkier and heavier. If you're trying to save weight on your backpacking trip, using a tarp is definitely not the way to do it.
Second, tarps don't usually fit as snugly as footprints. This means that there's more chance for insects and other small critters to get into your tent (not to mention rain and water).
Finally, tarps can be slippery, which isn't ideal when you're trying to sleep in your tent. A good footprint will have some sort of grippy material on the underside so that it stays put when you put your tent on top of it.
So while you could technically use a tarp instead of a footprint, it's really not the best idea. Stick with a purpose-made footprint for the best results.