Isle means island, but you almost never use it in everyday talk for a piece of land in the sea. You meet it in poems, older writing, and names: the British Isles, the Isle of Wight, the Isle of Man. It sounds like “I’ll”; the s is silent. Though actually, isle and island look like sister words, and they are not. Isle comes from French, from Latin insula. Island is a different old English word; people later stuck an s into island because of isle. So the spelling is tidier than the history. In names, keep isle if that is the official form; in a normal sentence, island is the word you want.