An animal model is a non-human species used in medical research because it can mimic aspects of a disease found in humans. Animal models are used to obtain information about a disease and its prevention, diagnosis, and treatment. By using animals, researchers can carry out experiments that would be impractical or ethically prohibited with humans.
Animal models are actually a critical part of biological research, because if you imagine, experimentation on humans is at least frowned upon and actually really more likely prohibited in most of the kinds of things that we want to do. We need a stand-in of some sort, so scientists rely on the conservation of processes, biological processes, that occur across all species, or some species, and we use a variety of them. The most common ones are the yeast saccharomycetales (the same one you use in bread), fruit flies (drosophila), nematodes, mice, zebrafish, and there are a few others like rats that are often used as well. And they're all stand-ins that we use to study the kinds of questions that really are impossible to do in humans.