The population of a coin refers to how many coins have been certified by a given grading service in that particular grade. The amount higher refers to how many have been certified in a higher grade by that grading service.
The population data corresponds to the particular coin date, variety, and mint. For example, an 1881-S S$1 Morgan silver dollar graded MS68 currently has a population of 48 coins with only 1 being graded higher. PCGS and NGC provide periodic reports of population data and we always use the latest figures available in giving our descriptions.