Complex sentence: Theoretically, there are 2 m possible Boolean functions of these "m" signals, but the structure of the ROM allows just n of these functions to be produced at the output pins.
Split into simpler sentences: Theoretically, there are "2 2 m" possible Boolean functions of these "m" input signals. Note that the structure of the ROM allows just "n" of the "2 2 m" possible such Boolean functions to be produced at the output pins.