Sentience is the ability to experience subjective feelings or perceptions. At least, that's the simplified definition. The more complex one is still being debated by philosophers who seem determined never to agree on anything. In essence, sentience is used to draw a line between intelligent beings who are entitled to certain rights and dignities, and all the others that can just be abused without guilt.
Application of the term in individual cases is often arbitrary and difficult to determine. Many times, beings have been used as slaves or treated like common animals because they were wrongly determined to be non-sentient. There are even some who insist that a given species can hold an entire spectrum of sentient and non-sentient members, as seems to be the case with Cattle.