In plain words
A moral patient is an entity whose treatment matters morally for that entity’s own sake. Moral patienthood, also called moral patiency, is the status of being such an entity.
A closer look
The central question concerns what others owe the entity. Many theories give moral importance to the capacity for experiences such as suffering or well-being; other approaches emphasize different properties or relationships. The grounds and degree of moral status are debated. Moral patienthood does not require the ability to explain moral rules or accept responsibility.
Applied to AI, the issue is whether a system could have morally relevant interests or experiences, and what evidence would justify that conclusion. Fluent language, task competence, and statements about feelings do not settle it. Neither the criteria nor their application to AI have a universally accepted answer.
In practice
A sentient animal is commonly regarded as deserving protection from suffering even though it is not expected to understand and fulfill human moral duties. This illustrates patienthood without full moral agency.
A useful distinction
“Patient” here does not mean a medical patient. It also does not mean every AI deserves rights, or that moral status must be identical across all beings. Those are further questions requiring separate arguments.