This dialog allows to modify velocity, note length, note pitch and note start. We'll explain this dialog by an example: Select length as 'Source' and velocity as 'Destin' and select some tracks in the trackwin. In the lower part of the dialog you can paint some kind of curve now, see section Random Rhythm for details.
When you press the Apply-Button the following happens: for every note on event, JAZZ looks at its length and looks up the length on the x axis of the curve. Now it looks at the curve and finds the corresponding value at the y-axis. This value is assigned to the velocity of the note on event. So, if you draw a diagonal line from bottom left to top right, short notes will get a low velocity and long note will get a high velocity.
For rhythm this is analogous: the mapper looks up the start time in the x axis and takes the value from the height of the corresponding bar.
When using random as 'Source' things are a bit different. The y-axis shows probability values now and the x-axis shows possible Destin values. JAZZ chooses for every note-on a new destin value - one of all the values shown on the x-axis. But those values, with a high probability are chosen more often, values with probability 0 will never be chosen.
The switch 'Add value' means, that the Destin value is added to the existing value and the value is not replaced. You can subtract values too by adding negative Destin values.
The rhythm and random features are probably the most interesting. The rhythm allows, to adjust velocity to a certain groove and random allows to 'humanize' midi events, add some very small timing inaccuracy or velocity changes.