Sentience is the capacity for experiences with a felt character, including states that can be pleasant or painful. Its ethical importance lies in the possibility that things can go better or worse for an experiencing subject. It differs from intelligence, language, self-recognition, and being able to make moral judgments.
The ethical argument starts with the possibility of a subject for whom an experience is good or bad. Intelligence or useful behavior alone does not establish that capacity, and lack of sophisticated language does not rule it out. Moral standing then concerns whether those experiences deserve consideration. This separates an empirical question about experience from a normative question about its significance. Neither can replace the other: a theory needs relevant evidence and a reason for taking the evidence seriously.