When a pianist plays a passage marked grandioso, they play more slowly and with loud, heavy chords, so it sounds majestic. Singers in a choir can sing grandioso, too, especially to end a piece with a grand, majestic, awesome sound.
Grandioso is Italian for "grand" or "majestic." We have a similar word in English, grandiose, but this word usually has a slightly negative vibe, suggesting that something is trying to sound grander or more impressive than it actually is. In music, a truly impressive, grandioso sound is typically achieved by slowing down, increasing the volume, and adding more harmony. To remind an orchestra to play grandioso, the conductor usually uses large arm movements. The word grandioso can be used as either an adverb or an adjective.