What kind of bug is black and looks like a roly poly with 2 yellow stripes?

The bug you're describing is likely a type of pill bug or roly-poly known as a "striped pill bug" or "banded pill bug," which can sometimes have distinct markings. These critters belong to the family Armadillidiidae and are crustaceans, not true insects. They often have a segmented, armored appearance and can be found in moist environments. The yellow stripes you mentioned may vary in appearance depending on the specific species or individual.