What does Bob Ross mean when he says liquid white?

When Bob Ross says liquid white, he means a thin, oily base coat that he applies to the entire canvas before starting to paint, which is key to his wet-on-wet oil painting technique. Liquid white is essentially a thinned version of white oil paint, made from titanium white pigment mixed with linseed oil and other mediums to make it very fluid and slow drying. By coating the canvas with a thin, even layer of liquid white, the canvas stays wet and slick, allowing Bob Ross to blend colors easily, create soft clouds, mountains, and happy little trees by mixing colors directly on the canvas with large brushes and palette knives. The wet base allows colors to mix and glide without dragging. It is similar to traditional oil painting technique of imprimatura but wetter. Bob Ross sold his own brand of liquid white, but you can make similar by thinning titanium white oil paint with linseed oil or liquid clear medium. Liquid clear is similar but clear instead of white, used when you want dark background. So liquid white is wet white oil medium base coat for wet-on-wet blending.