In a nutshell:
The classic blackcurrant fruit crème liqueur.
Boudier are based in Dijon the spiritual home of Crème de Cassis and theirs is one of the best around. Ripe blackcurrant are macerated, without preservatives or colouring, before the addition of a small amount of sugar.
Kir Royale made with Crémant de Bourgogone always springs to mind as the perfect way to mix this.