A coletilla is a short ending that the singer tags onto a letra. The singer often repeats it like a little chorus (estribillo). Here's a common one por alegrías:
Una coletilla por alegrías
Popular
Que me lo tienes que dar
el tacón de la bota
que para taconear*
You have to give me
the heel of your boot
to make music with my feet
You can see a video of María la Sabina sing it here: