FF 51 03 tt tt tt

Indicates a tempo change. The 3 data bytes of tt tt tt are the tempo in microseconds per quarter note. In other words, the microsecond tempo value tells you how long each one of your sequencer's "quarter notes" should be. For example, if you have the 3 bytes of 07 A1 20, then each quarter note should be 0x07A120 (or 500,000) microseconds long.

So, the MIDI file format expresses tempo as "the amount of time (ie, microseconds) per quarter note".

NOTE: If there are no tempo events in a MIDI file, then the tempo is assumed to be 120 BPM

In a format 0 file, the tempo changes are scattered throughout the one MTrk. In format 1, the very first MTrk should consist of only the tempo (and time signature) events so that it could be read by some device capable of generating a "tempo map". It is best not to place MIDI events in this MTrk. In format 2, each MTrk should begin with at least one initial tempo (and time signature) event.