An arcane focus is a type of Spellcasting Focus used by some arcane casters so they can cast certain types of spells.
For an arcane caster to cast a spell with a material (M) component, a spellcaster has three options.
They can use either the the spell components, or a component pouch, or a spellcasting focus. To make spellcasting as easy as possible, it’s usually best to use either the pouch or the focus.