A prawn cracker is the Southeast Asian snack made from starch and prawn, dried and then deep-fried until it expands into a crisp, airy cracker.
The usual dough combines tapioca or another starch with ground prawn, seafood paste and seasonings. It is rolled into a log, sliced, dried and stored in a hard raw form. Deep-frying rapidly expands the dried slices, producing the light texture associated with the finished snack.
The food has many regional names. In Indonesia it is commonly called krupuk udang; in Malaysia, Singapore, Brunei and southern Thailand it is often called keropok udang. Related versions include Filipino kropek and Vietnamese bánh phồng tôm. Recipes vary in the amount and type of seafood, and some crackers use fish or other flavourings instead.
That variation explains a common mix-up: “fish cracker” can describe a related product, but the question’s combination of prawn and starch points specifically to prawn cracker. Poppadoms are a different South Asian fried or baked snack, generally made from pulses or rice.