B/X Companion is a companion volume to the 1980 edtion of Basic and Expert D&D edited by Tom Moldvay.
The companion level book for the Moldvay edition of Basic D&D was never released. The newer Mentzer edition of D&D replaced the Moldvay edition and companion level materials were specifically added to that edition via the Companion set and the Rules Cyclopedia.
Here is a bullet-point list of the material added for Moldvay D&D by the B/X Companion:
Character levels 15-36
70+ new monsters designed for high level play.
100+ new magic items plus an entirely new (not present in Basic D&D) class of magical items.
Spell tables up to level 7 for clerics and level 9 for magic-users.
58 new spells.
Additional abilities for high level characters.
The B/X Companion also provides rules for running Dominions, commanding armies and engaging in mass combat, and adventuring in hazardous environments and other planes.