They're essentially a part of a person's being, they're a source of a person's abilities and skills. They also include a small pocket dimension in them and are commonly used for storage. The symbols on them are unique to each person, each ranging from an everyday math symbol or an entirely made up scribble, some say the symbols can tell a person's personality. (some does card readings, which are basically horoscopes and stuff. it's more difficult since the symbols are unique.)

The symbols are mostly for identification too, and are also a formal way to greet each other. Cards can easily be summoned by a certain flick of a limb and the intent to summon, most are taught from their parents at a young age even before learning to speak.

No one knows why the exist, or where's the source since the flower doesn't know either."

The butterfly smiled

"You know though."