Albedo is a measure of how much sunlight (solar radiation) a surface reflects. It is expressed as a percentage or a decimal between 0 and 1. A high albedo means a surface reflects most of the sunlight, while a low albedo means it absorbs more heat.
For example:
Snow and ice have a high albedo, reflecting most sunlight, which keeps them cooler.
Asphalt and concrete have low albedo, absorbing more heat and contributing to the Urban Heat Island effect.