The most important thing the General MIDI Standard does is define what instrument sound is heard when a specific Program Change index is used. Without General MIDI you might play a string quartet file and it would be heard on an accordion, trumpet, and kazoo. By using the General MIDI standard program numbers you will hear a violin, viola and cello.
General MIDI also defines channel 10 as a "Drum Channel". A set of drums is assigned to this channel. Individual drums are selected by playing specific notes.
Channel Priority and Voice Stealing
A "voice" is something within the synthesizer that can play one note. A triad chord requires three voices. Synthesizers generally have a limit to the number of voices they can play depending on the power of the computer. Generally you can count on having between 8 to 32 voices available. Mobileer software can support any reasonable number of voices. The maximum is set by the manufacturer of the device. Please check with the manufacturer to see how many voices they allow.
If your song exceeds the number of available voices then older voices will be stolen so that new notes can be played. You often cannot hear this happening but sometimes you can hear an old note cut off prematurely.
General MIDI assigns priority to each channel so that new notes will steal voices first from the lower priority channels. You can arrange your instruments on specific channels so that the most important sounds have a higher priority and will be less likely to be stolen. The highest priority channel is 10, the drum channel, followed by channel 1 to 9, then 11 to 16. 16 is the lowest priority channel. You might, therefore, want to put your important lead melody on channel one, and unimportant ornamentation on channel 16.
Instrument Numbering - Humans versus Computers
Humans like to number things starting with one. Computers like to number things starting with zero. This is a source of great confusion when referring MIDI. In the actual MIDI byte stream, instruments are numbered between 0 and 127. But many keyboards and software programs number instruments between 1 and 128. They subtract one or add one as needed. One way to tell how your device or software application works is to try to play the first instrument, the Acoustic Grand Piano. Note whether it is labeled a zero or one.
A list of the instruments and drums in General MIDI are defined here. Note that this chart labels the instruments from 1 to 128.
Notes on Instruments
Instruments 81 to 104 are abstract Lead, Pad and FX instruments. They are unusual sounding instruments like "atmosphere". There is no clear standard for what "atmosphere" sounds like so the sound of these instruments may vary significantly from one synthesizer to another.
Instruments 121 to 128 are sound effects like Helicopter and Gunshot. They generally do not respond to pitch so do not try to play a melody on these instruments.