Ethology question about owls: What does "Clack clack clack" mean? Also, what does eye blinking mean in a GHO?

{Great horned owl} I volunteer at a wildlife rehab center and we have a bunch of wild ones there, plus a few that are educational animals because they can't be released into the wild. I'm guessing that the clacking is a threatening sound, but I'm stumped about the eye blinking. My cats blink in a slow and sultry manner to show love. Why do owls blink at you?