An ‘unusual’ grouping of notes within a bar. For instance, notes are grouped in twos or in fours within bars in 4/4 (that would be the time signature where there are 4 quarter notes within a bar). So, if we want to fit 3 notes where 2 notes would have been, we notate it with triplets. If we want to fit 7 notes where 2 notes would have been, we would notate it with septuplets. Funky names, right?
And the silly part? We notate it by literally writing 13 notes where only 5 should have fit and adding brackets on top (or below) with the number 13. So, it’s like telling us to go fun ourselves. *Did it come through as passive-aggressive or something? ‘Cause that’s exactly what it was -what it is.*