According to Fear Free Happy Homes, "Cats are constantly communicating their feelings and intent with humans and other animals by using their primary communication method - body language. The problem is, cat communication signals can be so subtle, it's easy to miss or to misinterpret what a cat is trying to say. As a result, when a cat's polite warnings go unheeded, their gentle requests may escalate into louder shouts to get others to listen (think a yowl, hiss, or swat of the paw).
Learning the must-know cat communication signs, including the initial whispers of fear, anxiety, and stress as well as the key signs of calm contentment is protective. Learn ways to better understand your cat in the following cat body language guide for safe, happier interactions and strengthened feline friendships."
Source: https://www.fearfreehappyhomes.com/kit/cats-101/