The biological concept defines a species as members of a population that actually or potentially interbreed in nature… (source UC Berkeley’s Understanding Evolution ).
Forr example the two species of orangutans Bornean and Sumatran (Pongo pygmaeus and Pongo abelii) would not be considered one species because they inhabit two distinct islands. Orangutans don’t swim thus they would never mate in reality and are not considered one species according to the biological species concept. They can and have interbred in zoos. Yet because they are on two distinct islands in nature one could argue that they are two separate species that would never reproduce.
Now hamadryas baboons and olive baboons are considered two different species due to their genetics ( Papio hamadryas and Papio anubis) but they can and do occasionally interbreed. Thus according to the biological species concept you could consider them one species. They come into contact with each other in nature and they do interbreed.
Lions and tigers can reproduce in captivity and some of their offspring are even fertile themselves. On the one hand you could argue that this means the two are one species. They could potentially interbreed in nature under very extreme or unlikely circumstances. As you can see the biological species concept is not without is flaws. Sometimes it is very useful and other times it’s not as suitable as other ideas.