The concept seems pretty clear to me from a quick look on wikipedia

https://en.wikipedia.org/wiki/Sentience

Sentience is the ability to experience feelings and sensations. [3] It may not necessarily imply higher cognitive functions such as awareness, reasoning, or complex thought processes.

Also further down:

Animals such as pigs, chickens, and fish are typically recognized as sentient. There is more uncertainty regarding insects, and findings on certain insect species may not be applicable to others.

And if you don't like wikipedia here is the Stanford Encyclopedia of Philosophy

[...] we raise cattle, chickens, and pigs. Evidence from their behavior, their brains, and their evolutionary origins, adduced in Allen 2004, Andrews 2016, and Tye 2016, supports the view that they have mental lives and, importantly, are sentient creatures with likes and dislikes. Even chickens and other “birdbrains” have interesting mental lives. The exhaustive Marino 2017 collects evidence that chickens can adopt others’ visual perspectives, communicate deceptively, engage in arithmetic and simple logical reasoning, and keep track of pecking orders and short increments of time. Their personalities vary with respect to boldness, self-control, and vigilance.