RP-003_General_MIDI_System_Level_1_Specification_96-1-4_0.1.pdf
The "General MIDI System Level 1" specification -- also known as "GM", "General MIDI 1" and "GM 1" -- defines specific features of a MIDI sound generator (synthesizer), primarily so that MIDI files are shareable.
Without General MIDI, playback of MIDI files created on one MIDI instrument might sound totally different on a different MIDI instrument, because sound selection in MIDI is done by "Program Number", not a description of the sound. The GM specification assigns specific sound names (such as "Electric Piano" and "Oboe") to each Program Number, but the acoustic characteristics of the each sound are not defined.