It's changes in gene frequency within a population.
Microevolution occurs when there are changes in gene 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: genetic drift, natural selection, gene flow, and mutations.
Macroevolution is evolution on a large scale, a scale that encompasses more than just the species level.
When we're referring to how the demise of the dinosaurs left room for mammals to evolve or how chimpanzees and humans last shared a common ancestor 6-7 million years ago, we're talking about macroevolution.
Trends in evolution or other large-scale examples, the evolution of amphibians for example, are parts of macroevolution. This is in contrast to microevolution, or evolution on the smaller scale of populations.
To learn more about the subject, check out this Nature page.