The albedo is simply the ability of a surface to reflect the Sun light. It differs from one surface to another, with the more barren lands of the ones covered with ice being able to reflect more Sun light, while the ones that are covered with dense vegetation reflecting less.
The prairies are covered with grasses, so they are kind of in between the ones that reflect more and less. While the grasses manage to reflect part of the sunlight and not allow to get to the ground, because they are short and there's spaces between them, still part of the Sun light manages to get to the ground.
To compare it with the tundra biome, the prairie reflects much less. The reason for that is that the tundra is covered with ice for most of the year. The ice had very light coloration, and the light colors have much higher albedo. The ice manages to reflect most of the Sun light, which i turn causes less heating on the surface, and the ice expands even more.