As an English surname (there could be other sources) it's basically the word isle, i.e., island. Someone who lived on an island, or just a raised up dry patch in wetland. Our word comes from the French, who were also responsible for Lisle (l'ile). Another word of the same meaning is the Old Norse holm, which gives surnames such as Holme and Holmes. Then there's Inch and Ince from the Celtic. There must have been a lot of places that were identified as islands.