This is linseed oil, extracted from flax and used since antiquity for oil colors. It is an essential material for any oil painter, useful for making paint more glossy, transparent, and brushable. It can be mixed in different proportions with solvents like Gamsol or turpentine to create different textures in oil painting.
Holbein's purified linseed oil is a lovely pale yellow that, when mixed, imparts virtually no color to paint. However, adding large quantities of linseed oil can speed up the yellowing of oil colors, particularly light colors. Poppy oil is a non-yellowing option that you may wish to use with lighter colors for improved permanence.