You summon an elemental jann, a minor embodiment of one of the four elements that takes the form of a tiny animal of your choice (both are chosen when you first cast the spell and are permanent for all subsequent casting). This creature has the statistics and capabilities of the animal form you chose but has no special attack forms (poison, rend, etc.). In combat, it deals +1d4 damage of a type appropriate to its elemental nature:
Air - Electric
Earth - Thunder
Water - Cold
Fire - Fire
At any point during the spell's hour duration, you may expend a spell slot and command your jann to 'fetch' knowledge of a spell of that slot's level or lower for you. This immediately dismisses the jann and ends the spell. The jann will take 1d10 minutes per level of the spell requested to return and during this time, you cannot cast conjure jann or any spell involving your chosen elemental type or the extra damage type your jann inflicts. The jann can retrieve knowledge of any spell that exists on your current plane of existence, but you must request the spell by name and have at least a basic understanding of the spell's effects.
The jann may be unsuccessful; the DM will make an Arcane check for the jann (using your bonuses) with a difficulty equal to 10 + the spell's level + 1d10 + 5 if the spell is not available to your class + 5 if the spell is not of a type you can cast (divine or arcane). If the jann is unsuccessful, the spell slot spent in the attempt is still lost.
If the jann succeeds, it returns and you gain the ability to cast the spell without needing to spend another slot to do so. All other limitations and requirements for casting the spell apply (casting time, material components needed, etc.). You retain the ability to cast this spell until you do so once, you are reduced to 0 hit points, or you take a short or long rest. While you have the ability to cast this fetched spell, you cannot send your jann to retrieve a new one (though you can summon your jann for other reasons).
If you have your jann retrieve a spell that you have on your class spell list and would be capable of casting normally, you can forgo casting it and instead have your jann scribe the spell into your spellbook or ritual book (if you have one and can benefit from one). This service requires you to spend an additional spell slot of the appropriate level but does not require extra materials or even the usual cost in ink and supplies. You can use your jann in this way on behalf of another caster, in which cast the retrieved spell to be scribed must be a spell your intended beneficiary has on their class spell list and be able to cast normally.
Jann can also be used to transfer a spell from one spellbook to your own (or a beneficiary). The target spellbook cannot be in the possession of another living creature and must be within 60 feet and line of sight. When cast for this purpose, all the limitations for scribing noted above apply but there is no Arcane check required. The spell is automatically 'fetched' before scribing.
Regardless of the purpose to which it is used, you may only have a Jann retrieve a spell once per long rest.