When food is crispy, the outside is pleasantly hard and makes a little sound when you bite it. Fried chicken skin, roast potatoes, bacon, pastry, and a well-cooked spring roll can all be crispy. It is a good word: people want that outside, not a soggy one. The word stays close to food. American English uses it a lot in the kitchen; British English uses it too, but also uses crisp for some of the same food ideas, and crisp has other jobs (cool weather, a clean sharp style, a firm manner). Crispy does not really do those other jobs. It is informal-to-neutral, the kind of word you hear at the table, not in a science book. An older English use of crispy meant curly, like tightly waved hair, but the meaning learners meet now is almost always this pleasant hard outside on food.