Tread Depth

I have a 1988 Mazda 929 with a 3.0L Fl V6 engine. It's an automatic and has 222,996 original miles. I just bought the car (which had a new engine put in) early this year. I had alignment problems with my other car that was the same year and make. I took it to get the oil changed and everything checked. They said my tread depth was as follows:
Driver Front - 9/32
Driver Rear - 8/32
Passenger Rear - 7/32
Passenger Front - 7/32
Does anyone know what this means or indicates?