Available in several sizes, This tarp made by Grizzly Tarps is perfect for placing under your tent to prevent damage to the bottom of your tent and to prevent moisture from finding its way into your tent. Many tent do a pretty good job of keeping out water but extra protection is really nice. These tarp are also good for keeping your tent clean and free of dirt and mud, which makes putting the tent away after camping a bit less messy. It saves you from getting dirt everywhere the next time you unpack your tent.
These tarps are also good for placing inside your tent on the tent floor. The purpose of this is to make clean up of the dirt you would track inside the tent much easier.
Another benefit is to stretch it out between some trees or poles to provide a shaded area for your camping trip, beach vacation, and even just around the yard. Nothing like some extra shade in the middle of the summer, especially while cooking and eating. It could also work as shelter from rain. In a bind it could function as a makeshift rain fly.
Some people use these for covering wood piles. Others for securing cargo in the bed of there pickup truck. Being waterproof they will keep your firewood or your cargo dry from the rain.
The built in grommets are there for tying the tarp to whatever you need to tie it to.
These tarps are available in several sizes. Check out Amazon for a good price on these reliable tarps.