Retread just means to put new rubber on an old tire. They still do it on big trucks. The saying here means he is going to school for something better than just to get a lousy job repairing tires.

The grooves in a tire are called the "tread." When the tires are old and the tread is gone, you can cut into the tire and form a new tread. You can only do this with very large tires for trucks. It's called "retreading".