The format type can have values of 0, 1, or 2.
A value of 0 means that the MIDI file contains one track chunk with all relevant MIDI events, including meta events with information about the MIDI song as well as musical MIDI events that actually do something.
A value of 1 means that the MIDI file contains two or more track chunks (up to 65,535). By convention the first chunk will contain meta events with information about the song and the remaining chunks will contain the musical MIDI events.
A value of 2 also means multiple track chunks, but in this case each track represents a separate MIDI sequence, which may or may not be played simultaneously, and which may contain its own meta and musical events.