What does it mean when there's a flashing light on the dash that looks like an exclamation mark on a 2000 Ford Taurus SES sedan

Tire could be low

But I checked and they put air yesterday and today at the tire shop they recommended to change the back tires to the front and the front tires too the back.they told me the tires are still good to use them... once I left the tire shop, i was driving and the exclamation mark started turning on again....

I know this is a little old but the light I believe you are referring to is Transmission. Could be low on fluid , have a sensor issue or another internal problem. Check the fluid, should be a reddish color, check for metal flakes and/or a burnt smell. Take the car to a transmission shop and have them retrieve the codes to properly identify the issue. Transmissions are pretty common with Taurus.