M.S.
They could mean either one. Typically babies will roll from belly to back first because its easier to do (and because they hate tummy time, LOL). Rolling from back to belly takes more strength and coordination to do, and usually happens a month or two AFTER they can go from belly to back.
A 2 month old might be able to do belly to back, but probably wont be able to do back to belly until 3 or 4 months.