Hi Bill,
There are two parts: the crawler and the mobile launcher platform. The crawler brings the mobile launcher platform out with the rocket. When it arrives at LC-39A or B, the crawler lowers the mobile launcher platform on to some pylons. The crawler withdraws a mile away and after the launch, comes back to lift the mobile launcher platform to take it back to the VAB.
Check on youtube, there should be a lot of Saturn V and Shuttle crawler videos that show the process.
SM