Tires are the only thing touching the road. The grooves in the tread move water out from under them, so worn tread means less grip in rain and snow and longer stops.
Want to check your tread at home? Use the penny test. Slide a penny into the tread with Lincoln's head pointing down. If you can see the top of his head, the tread is getting low.
Pressure matters as much as tread. Too much or too little air makes tires wear unevenly and can make the car handle worse. Your car's correct pressure is printed on a sticker inside the driver's door jamb, not on the tire. Check it about once a month. In winter, check more often, since cold air drops the pressure.
New tire prices depend on your tire size, the kind of tire and how many you're replacing. Book online and ask for a loaner, which is complimentary with a scheduled appointment.