You’ll see it a lot on the internet; cat lovers discussing how their cat does the classic ‘slow blink’ at them and happy to say that it is a sign that their cat loves them. In general people consider the cat slow blink to be a sign of affection from their cat.
It is, in a way but I think its origin indicates that its meaning can be slightly different.
The blink stops a cat staring. And a prolonged stare is intimidating to another cat particularly a subordinate one. The subordinate cat might withdraw.
Therefore in order to maintain social relationships and the benefits that come from that, non-aggressive cats when staring at other cats and their human companion will blink which is a signal that they are not hostile. The blink breaks up the stare.
To be non-hostile is not quite the same as being affectionate. However, it probably has evolved in human households to mean that.
Please search using the search box at the top of the site. You are bound to find what you are looking for.