Jinn traditionally are magical beings (beings of essential fire) from the Dusky Lands. We assume they're stuck in jars because Solomon vacuumed a bunch up (angels too) and stuffed them into objects, sometimes geasing them so they are bound to serve.

This is how King Soli got his civil works projects done.

There are tens of thousands of Solomon's captured Jinns, spread asunder and lost to time. But beware opening a vessel with Solomon's seal. Some are untamed and with just kill you, while others are tricky and will spoil any requests.

Those that are bound usually are limited only by their own endurance and patience. It's fairies who like to number their boons.