What is the grand staff used for in music theory?

A grand staff, which is two staves combined, is generally used for piano and keyboard instruments to denote which hands should be playing which notes. The top staff is marked with a treble clef, and the bottom staff is marked with a bass clef.

Clefs on a staff signify which notes are on the staff. For the bass clef, which represents the left hand, the line notes from bottom to top are G-B-D-F-A, and the spaces represent A-C-E-G. On the other hand, the treble clef signifies that the lines represent the notes E-G-B-D-F from bottom to top, while the spaces spell F-A-C-E.