An immobile, glimmering sphere forms around you. Any spell of 5th-level or lower cast from outside the sphere can't affect anything inside the sphere, even if it's cast with a higher level spell slot. Targeting something inside the sphere or including the globe's space in an area has no effect on anything inside.
Using a Higher-Level Spell Slot. The barrier blocks spells of one spell slot level higher for each slot level above 6th.