Footprint?

The footprint is a heavy bombproof ground sheet under your tent that has connectors for your poles. Most stores will try hard to sell it to you (for an extra $40 or so), claiming it's necessary protection for your tent floor. It also adds an extra pound or more to your tent weight. The only place such a footprint might be useful is if you want to take the fly and footprint without the inner tent. In that case, you'd be better off taking a lightweight tarp and separate light weight ground sheet.

I haven't used a ground sheet for years, except for camping in northern California where spores of sudden oak death disease may be in the soil (I don't want to bring the disease north to Oregon, where it's a real threat to our large plant nursery industry). In that case, I use a piece of 1 mil painter's dropcloth, cut to fit. The plastic sheet should be slightly smaller than your tent floor so it doesn't lead water under the tent.

Don't let the store sell you a footprint; you definitely don't need it! Just use reasonable care removing sticks and small stones from the site before you put the tent up.

The one additional thing you need to watch out for, and where a ground cloth of some kind is useful, is when you're camping where there are lots of pine needles. They can poke pin holes in your tent/bivy bottom easily.