Have you ever glanced over to your cat and seen them narrow their eyes and slowly blink at you? Slow blinking by a cat is a sign of trust and affection.
What is slow blinking by a cat?
When a cat slow blinks, their facial expression follows a certain pattern. The slow blink sequence starts with the cat having a neutral expression. The cat then lowers their upper eyelid into a half blink position. The cat then closes their eyes while narrowing the eyes before slowly opening their eyes again.
You cat may do this slow blink sequence once or repeat it multiple times.
What does it mean when a cat slow blinks?
Researchers hypothesize that cats slow blink when they are happy.  Cats slow blink at their owners as a sign of trust and affection.
Cats who feel threatened will keep a laser-like focus on the threat. By relaxing their focus and even closes their eyes in front of someone, cats are sending the signal that they don’t feel threatened by that person.
Some people refer to a cat slow blinking as a “cat kiss” as they feel their cat is essentially blowing them a kiss as if to say, “I love you.”
People can communicate with cats by slow blinking
The body language of slow blinking is a two-way street with cats.
Researchers measured the response of cats to their owners in one study and found that when people slow-blinked at their cats, the cats often slow-blinked in return. 
Slow blinking can help owners bond with their cats to demonstrate their affection for their cats.
Slow blinking can also be used on cats you come across while out to communicate. The same study mentioned above also found that cats were more likely to approach strangers when those people engaged in slow blinking. Slow blinking is a way to project that you mean no harm to a strange cat as opposed to a hard stare which is received as being aggressive by cats.
 Humphrey, T., Proops, L., Forman, J., Spooner, R., & McComb, K. (2020). The role of cat eye narrowing movements in cat–human communication. Scientific reports, 10(1), 1-8. https://doi.org/10.1038/s41598-020-73426-0