Given that other officer ranks command the same unit groupings as their modern day U.S. counterparts, a Major General probably commands division sized units. This has yet to be observed in the Halo universe though. They may also serve as high-level staff officers at major commands.
In informal settings, Major Generals are typically referred to simply as "General".
A Major General may be also referred to as a "two-star General" to distinguish it from the other ranks of General; so named for the two star rank insignia worn on the uniform.