It’s changes in gene frequency within a population.
Microevolution occurs when there are changes in frequency within a population over a short period of time. Microevolution specifically refers to changes within a population a population being a group of organisms that breed with each other.
Microevolution differs from macroevolution in scale. Macroevolution occurs in a large scale: when we’re talking about evolution beyond the species scale we’re talking about macroevolution. The mechanisms of the micro and macroevolution are the same: gene flow and .
To learn more check out this excellent resource from Understanding Evolution.