When asked what "swing" was, Louis Armstrong once famously said (words to the effect) that if you didn't know, then no amount of explanation would do you any good. Nevertheless here is an illustration of one of the aspects - one which allows midi play-back to give some kind of an imitation of the real thing.
As an example of a well known "swung" piece, let us take Paul Desmond's "Take Five". There are two themes. The first may be written essentially as:
Each of the play-back buttons below plays it back in a different (albeit mechanical) way:
Play-back button 0: plays it straight - with absolutely classical quavers (eighth notes).
Play-back button 6: (for example) plays every pair of quavers such the first is 9/16 of a crotchet and the second is 7/16 of a crotchet.
[In principle this one could be written with the first quaver replaces by 9 hemidemisemiquavers tied together, and the second by 7 of the same.]
It is important to note that the most important aspect is not the different durations of the quavers (that can be amended by using staccato and tenuto markings) but that the 2nd and 4th quavers of a group of 4 fall slightly later than where they would when the piece is played classically.
From left to right, the buttons gradually increase the quaver ratios as shown in brackets until the last is what happens when you play a quaver pair as a crotchet-quaver triplet.
Play-back button 16: triplet feel:
In order to attempt to direct the player to swing a piece one often sees the instruction depicted right. If taken literally this
results in the style of the right-most play button above which, in this author's experience, is almost always highly undesirable and nothing at
all like jazz.
Different pieces and different jazz styles can be mimicked with the computer by different options on the swing style above. I think this particular piece sounds best somewhere in the range from button 6 to 10.