Sentience just means the ability to perceive and feel things. It implies consciousness, but that's debated. You may be thinking of sapience, which is usually considered to comprise of a higher range of cognitive functions such as reasoning, self-awareness and perhaps theory of mind although some animals seem to have that last one too.

Fundamentally what this does is further recognise in law that some animals are able to suffer, and that we have some sort of responsibility to avoid inflicting unnecessary suffering such as for entertainment or through neglect.