The concept of a species in biology is tricky and controversial. John Wilkins is an expert on the controversies surrounding the term and he rejects the notion that the species concept is a mere verbal convenience or that it is simply a theoretical object. Instead, species are real things in the world:
This brings us to the third alternative: species aren’t theoretical objects at all; they are objects that have phenomenal salience . That is, we do not define species, we see them. Consider an analogous case: mountains. Mountains are hard to define, and they have a multitude of geological causes, ranging from uplift, subduction, vulcanism, differential erosion, and so forth. “Mountain” is not a theoretical object of geology – subduction zones, tectonic plates, and volcanoes are. A mountain is just something you see, although there are no necessary sets of properties (or heights) that mountains have to have, and it is often vague when differentiating between them. A mountain calls for an explanation, and the explanation relies on theory, but equally so do mesas, land bridges, and caves.
So the suggested answer to the question: what is a species? is that a species is something one sees when one realizes that two organisms are in the relevant manner the same. They are natural objects, not mere conveniences, but they are not derived from explanations, but rather they call for them.