Most sporting good stores and treadmill manufacturers sell rubber mats designed specifically for use under a treadmill. Placing one of these mats under the treadmill will minimize damage to the floor below. The mat will keep the treadmill from making permanent indentations in your carpet, and if you have a hardwood floor it will keep the treadmill from damaging the finish.
If you have a hardwood or vinyl floor in your exercise room you may want to put a piece of carpet under the treadmill. In some cases carpeting can be less expensive than a formal treadmill mat, especially if you have leftover scraps from when you carpeted the other rooms in your house. The carpeting will keep the treadmill from scratching your hardwood floor or leaving cuts or marks on the vinyl. You also can use a carpet color that matches the rest of the room, making it more eye appealing than a mat made for treadmills.
If you live in an apartment or condo you may need to place soundproofing material under your treadmill. To soundproof your floor so neighbors do not hear the whirl of the motor or the sound if your pounding feet, you will have to tear up your existing flooring. You will then add a soundproofing material to the joists and original floor, then recover the base with a new carpet or flooring of your choice. Placing soundproofing materials under your treadmill can be costly and should be considered as a last resort.