An Arcane Focus is any handheld item that contains a well of magic from which a spell caster can draw power and channel magic through. Traditionally an arcane focus comes in the form of a wand, staff, or crystal, but it can really be any tool or handheld item that the caster has a strong connection or familiarity with.

An Arcane Focus holds untapped energy within that comes directly from a captured portion of a magical wellspring. Anyone who can learn to use a focus properly are able to cast magical spells with it.

Even when the focus is drained of its magical energy it will regenerate completely within a day. But even drained it's far from useless. If the caster has any magic stored within themselves from a magical gift or bloodline, the focus can use that energy instead. If it has access to any other magical source a spell caster can continue to cast spells using this item. But overuse can cause it to overheat and completely disintegrate.

Arcane Focuses have been in use almost since the discovery of magic itself!

This is not a workaround for the laws of magic. The Arcane Focus, no matter its design or attempts to make it otherwise, it will not increase the users magical ability beyond what they can physically handle. In other words, it provides a safety net to make sure the caster doesn't overextend their magical use and summon the Herald.

Collage of Elbion - Learning to Duel
The Schools of Magic [Elbion & Vel Anir]