MIDI (Musical Instrument Digital Interface) is a protocol designed for recording and playing back music on digital synthesizers. MIDI itself does not make sound — it is just a series of messages like "note on," "note off," "note/pitch," "pitchbend," and many more. These messages are interpreted by a MIDI instrument to produce sound.
After installing the app you will be able to open files downloaded from the web. Select the file and choose "Open in MIDI Opener" or "Open in...", then select MIDI Opener from the list of apps.
Load a MIDI file from Dropbox, Google Drive, Apple's iCloud Drive, etc. by tapping the folder icon. Note that the Dropbox or Google Drive app must be installed and iCloud Drive must be configured in Settings first.
Each General MIDI (GM2) sound library contains 128 virtual instruments, plus percussion kits. Download .sf2 and .dls files and add them to your list of available soundbanks in MIDI Opener.