Tassie cups is a term that means “small cups” in baking.
It can mean small tart shells made of pastry. Or, it can refer to a muffin pan with smaller than usual sized cups in it.
If a recipe says “Buy (or make) tassie cups”, it means pastry shells.
Otherwise, the recipe will specify something such as “foil tassie cups.”
The idea is to produce small, bite-sized tarts.
Tassie is a word in Scottish that actually means “cup”, as in from the French word for cup or glass, “tasse.”