Flipping a note means that the note-head is hung on the 'wrong' side of the stem.
The second and fourth notes are 'flipped' here. Often, if you want/need to work with this kind of notation, you'll want to manually adjust the individual note spacing (whitespace) as well.
Especially In piano and guitar parts where more melody lines have to be written on a single staff, you sometimes need this to correct the dots in chords that are at semitone distances. In most cases, MusiCAD detects overlap automatically.
With single notes, hanging the stem on the wrong side makes no sense. Instead, the note is shifted left or right relative to other notes in the score depending on the stem direction. The example below shows how two parts on a bar produce overlapping notes (f and g) (top picture) When note flip is used, the g shifts to the right and no overlap is visible anymore.
You can also do a 'note flip' with hotkey <ctrl-2>.