This is simply a tool used to cast spells with (PHB p. 151)
An arcane focus is a special item - an orb, a crystal, a rod, a specially constructed staff, a wand-like length of wood, or some similar item - designed to channel the power of arcane spells. A sorcerer, warlock, or wizard can use such an item as a
spellcasting focus, as described in chapter 10.
When casting a spell, they require a combination of 3 different "components" - Verbal (i.e the "spoken" component), Somatic (gestures of the hands, etc), and Material (a cricket's leg, or a tongue of a lizard). The arcane focus can be used in place of the material component in your spell (PHB p. 203, emphasis mine).
Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in chapter 5) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.