A cortina is a short piece (20–60 seconds) of non-dance music that is played between tandas

Tanda (tango)

A tanda is a turn of dancing in a milonga, and by association, a set of pieces of music, usually between three or five, that is played for one turn.Most commonly the music is tango, milonga or vals...

 at a milonga (tango dance event). The cortina lets the dancers know that the tanda has ended. The partners can then without insult thank each other and return to their own tables, to find a new dance partner at the next tanda

Tanda (tango)

A tanda is a turn of dancing in a milonga, and by association, a set of pieces of music, usually between three or five, that is played for one turn.Most commonly the music is tango, milonga or vals...

. Cortinas are used at many of the milongas in Argentina and Uruguay but are increasingly common elsewhere.