A tempo map is a part of a MIDI file. Musical events occur as a succession of events in time, whose speed is tempo. Music also organizes these according to a framework called meter, by partitioning time into patterns of "strong" and "weak" beats. MIDI's tempo map specifies the speed at which a file's events are transmitted within this framework: their tempo. If a file plays at a fixed tempo, its map is a horizontal line (e.g., measures 38 & 39 in this part of a MIDI sequencer’s display of the end of J.S. Bach’s prelude #8 from Book I of the Well-tempered Clavier):
But, if the tempo fluctuates as a function of time, such as in accelerando, allargando, or rubato, the line is respectively an upward- or downward-climbing one, or in the latter case, a complex curve, as seen here in measures 35 & 36 at left, or the ending in measures 40 & 41.
In most music, aesthetics demand that this flow of events not be linear, but fluctuate according to the expression the music intends to convey. Hence, a very emotional piece such as the preceding is excerpted from, although playing at a base tempo of MM=50 — which means 50 quarter notes per minute, holding back or lunging forward according to expressive impulse can result in a tempo map similar to that seen above: It flexes tempo down for the intermediate cadence at left, and again for the final at the right.
After recording a sequence in real time at a fixed tempo, or composing it using step-record mode, well-written MIDI sequencer software like MasterTracks Pro will have functionality to edit the MIDI tempo map, so as to result in a natural, expressive result, not a machine-like one, when playing the file.