"Licorice flavor" refers to the taste commonly associated with the black candy called licorice. It has a distinct sweet and slightly bitter taste, similar to anise or fennel. Full definition
Fennel seeds have long been used in baking and cooking, as the seeds have a sort of sweet licorice flavor. (annmariegianni.com)
I loved the slight licorice flavor that the fennel brought in paired with the sweet peas, and the lemon added a wonderful brightness to the dish. (spachethespatula.com)
My substitute attempt turned out delicious so I can only imagine that by using the called for fennel, the dish would result in an even deeper, richer experience with the mild - licorice flavor coming from the fennel. (epicurious.com)