What would cause this car problem?

My sister's 1999 Chevy Cavalier has over 190,000 on the car. It's in pretty decent shape. Tonight her car has an ETS OFF code on the dash. The weather has been very wet and messy. I went over to check out the car what I could find out. The tires on the car were really low. The lowest was 15 and 20 psi on the rear tires. What is the ETS and what would cause the light to come on? Is this the traction control system that comes on while driving on slippery roads? Thanks for any help.