This term refers to an accidental symbol that lowers a note by three semitones (or three half steps). This symbol is indicated by three flat (♭) symbols preceding the note. It should be noted that the triple flat is extremely rare and can only be found in a very few compositions throughout all of the history of modern musical notation. It is only used in classical music and is more theoretical than practical. Most musicians (professional or amateur) will never see or perform a triple flat in their entire musical career.
The triple flat symbol alters the pitch of the note to which it is attached as well as any subsequent occurrence of the same note (identical line or space) in the same measure. Notes with the same pitch name, but a higher or lower octave, are not effected. Any note with a triple flat that also has a tie across a barline carries the triple flat to the note on the other side of the barline. Notes in the new measure that are not tied to altered notes from the previous measure revert to their original pitch and are performed using the current key signature. It should also be noted that a triple flat will always be shown with three flat symbols (♭♭♭), regardless of the key signature. The example below shows a rare triple flat in the Piano Sonata No. 1 by Nikolai Roslavets, written in 1914. Notice that the flat symbols are connected at the top by beams and courtesy accidentals are also shown enclosed in parentheses marks.
Piano Sonata No. 1 (1914) - Nikolai Roslavets