hi there!!
so the short explanation of flairs is that it's a feature to decorate your posts with different colours and background that you may see across the forum (like mine, for example). to go over them individually:
main block - this is essentially, as far as to my understanding, containing the entire background of your posts. setting custom backgrounds i think is for supporters only, while regular users can instead set their post backgrounds to different colours.
avatar container - mostly self-explanatory, this has to deal with your avatar in particular and options to customise it. you can make your avatar circular or even put borders on it on the really basic level. you can get
really creative here.
your name - this is for customizing the username that appears on posts. setting custom username colours and fonts are examples of what you can do with it.
statistics - for customizing the stats that appear in your posts. basically customizing how your postcount and join date appear (or whether they don't at all!)
mini-biography - this is for customizing the mini-biography that appears in your posts (gender, last seen, last posted, location, etc).
that said, it's worth
looking up and learning CSS (if you aren't familiar already) as that'll help you a LOT with customizing your post flair and it's what is used to customize it in the first place. there are some restrictions as far as i'm aware that i can't think of off the top of my head, but it's a few elements that almost no one on the forum has used in a long time so you're probably fine using most css elements as long as they aren't too crazy.