An arcane focus is a magical item that helps the affected spell work.
An Arcane Focus should be an object around which the Protective Sphere of Protection from Evil and Good spell is cast in order to maintain it. It’s called out more explicitly because a lot of protective circles are not, so some DM’s might want to limit the availability of this clarifying sentence. A common arcane focus would be a stone (or something associated with Stone), but anything could suffice – even a small golden ring for those who have that movie idea stuck in their head. It just needs to be something substantial enough that it can keep up with occupying the entire 10′ diameter area around it for 24 hours at Level 1 or less, without being destroyed by any means. It cannot be moved, but it can be replaced with something else of equal or greater value if destroyed, or replaced magically by somebody who has the ability to do so – which is simply magic.
If you don’t want your wizards encircling themselves in tiny golden rings, there are other options that could work; perhaps they need to be chanting a specific incantation while standing on a specific rune made of a special kind of wood around which the spell was cast, or perhaps it must be held in one’s hand rather than placed upon an alter, or perhaps there is another arcane focus that is more commonly seen. In any case, you could make up any number of things to keep it well-grounded in your game world. And if you use the desire for arcane foci as an opportunity to develop or expand elements of your game world, or give clues about how magic actually works, that’s even better!
Let’s say you want to make it more challenging for wizards to cast protective spells like Protection from Evil and Good, or even spells like Detect Magic. Maybe you don’t want to simply not allow them at all – which is the simplest way to handle this – but instead wish to limit it more than that.
