The Shield Charm (Protego) was a charm that protected the caster with an invisible shield that reflected spells and blocks physical entities.

What spell breaks Protego Maxima?

This spell is the core of the shield summoned around Hogwarts; it is the shield itself that is seen descending over the castle. It should be noted however that on occasion protective enchantments can be breached with a shield penetration spell.

What can protego block?

It can be used to deflect both physical entities and spells. As an example of it blocking projectiles, Dolores Umbridge used a shield charm to protect herself from the centaurs' arrows in the Forbidden Forest.