The large leaves (up to a foot across) of the Hoja Santa plant have an unusual flavor that is hard to describe. It is a part of the peppercorn family, and so it has a slight pepper flavor as well as black licorice note as well.
Hoja Santa contains sassafras-like flavors and actually contains the same oils of the sassafras plant. There has been some question as to Hoja Santa's safety as the oils have been proven to cause cancer in animal studies, but there is no proof of this in humans.
Hoja Santa is a bit difficult to find in the United States, you probably won't find it in your local grocery store chain. If you live near a Latin community, the local "mercados" (markets) may have some. Also check for any local "Jardin Comunitario" (community garden.) You can also grow your own.