An islet is a very small island — a tiny bit of land in a sea, lake, or river. You see it in geography and in careful writing more than in chat. In everyday English people often just say a tiny island. It is a size word: not a full island in the usual picture, but still land, not only a rock that disappears at high tide (that is closer to a reef or a skerry, in specialist talk). The feel is descriptive, not poetic in the way isle can be.

小島；極小的島嶼