Answer:
There is some debate about the definition of a species, especially across fields. However, what it likely being discussed here is the biological species concept which states that:
An organism is considered a new species when there is reproductive isolation from the population that they originated from. In other words, for two organisms to be the same species they must be able to reproduce to create viable (able to survive) and fertile (able to reproduce) offspring.
Reproductive isolation can occur by different means, classified as being prezygotic or postzygotic isolation. These include things like mechanical, behavioral, temporal, ecological, and geographical isolation for prezygotic isolation. And things like hybrid infertility and hybrid inviability for postzygotic isolation. These are not relevant to the question but they give you an idea of what you could look up if you wanted more information.
