what does it mean when an anghle is equal to itself?
It is just another fancy way of saying "You are as old as you are," or "You are as tall as you are." No matter how old or how tall you are, you will always be the same age and height as yourself, now won't you?
Basically, it's like looking into a standard mirror (not a funhouse mirror) and seeing you, who will always be equal to you.
It is the reflexive property. Anything that is equal to itself is reflexive. It is not so much a=b because there is only one variable. So it is simply a=a, like looking in a mirror.