Magic Staffs are items that can greatly alter the spells that you cast, and some let you cast spells for free, without using Mana. However, you must pay a price in order to keep attuned with a Staff. Most Staffs will require you to spend Mana when you finish a Long rest to keep attuned with them, but others can require blood, or even your own soul as their price.

You must be holding the Staff with 1 hand in order to benefit from its effects, and you no longer need 1 free hand to cast Normal Spells while holding a Staff.