Liquid Clear is intended as a base coat, to be applied onto the canvas first before any color. This way you're immediately painting wet-on-wet, which is fundamental to the Bob Ross painting method.
These two are certainly not the only two mediums in the Bob Ross art supplies range -- the official Bob Ross website lists everything.
You can make a "generic version" by mixing up some white paint with whatever oil you use as a medium, or using the oil just as it is. If you want it to dry faster, add some turps or Liquin. Store in an air-tight container.
