The English idiom “cream of the crop” means the best of the best. The phrase refers to the person who is at the top of his or her profession, class, or art. Its literal meaning refers to the best produce of the harvest, and this meaning is still sometimes used, though the figurative use of the phrase is much more common in modern language.
The origins of the idiom are at least five centuries old. The actual idiom “cream of the crop” was first used in the 16th century, though using the word cream to figuratively refer to the best is likely even older. Cream is usually seen as the most desired part of the milk. It is the sweetest part and rises to the top of the milk. From that meaning of cream, the phrase entered common language to describe the best of the harvest and quickly passed into figurative use to describe the best in any category.
As with many English idioms, the "cream of the crop" entered the language when nearly everyone was involved in agriculture. At the time the idiom first entered the language, it would have been common for most people to keep a cow to provide milk for the family. This meant that nearly everyone would know the process by which cream, the richest part of the milk, rose to the top and was skimmed as a special treat or for making into butter.
Most people would have also been involved in growing produce to some extent. The gathering of the best produce from a year’s harvest would have been an event that was looked forward to all year. This meant that everyone knew exactly what the cream of the crop was and what was meant by the phrase when it was used figuratively.
The phrase’s popularity likely survived the ending of the agricultural lifestyle in part because of its alliteration. Alliteration occurs when the words of a phrase begin with the same consonant sound, such as cream and crop. Alliterations are easily remembered and will often keep a saying in common use when it otherwise would have faded into obscurity.