The 'cream' of anything, as in the 'cream of the crop', means the best or topmost. The word cream is used because in milk which has not been homogenized, the cream - the best part, for many - will rise to the top of the bottle. Anything that is at the top of some group or quantity would be like the cream in milk .

You may also see this stated as "cream of the crop". It basically means the best from the group.