Tina, "Cat Daddy" Jackson Galaxy explains it well, here:

https://www.youtube.com/watch?v=vdLf6I8oE-0

It's a way of saying "I love you." A slow, half blink while you look into your cat's eyes. I usually say I love you while doing it.

Try it a few times and don't give up. I guarantee you'll get a slow blink, back!