An ensemble, whether in A.I. or jazz, is a group that gets a better result than any of its members alone. The whole ensemble is better than the sum of its parts.
An A.I. ensemble refers to the use of several algorithms working on the same problem. The results of each of them are then combined into a single final answer. This approach has been found to be very successful solving real-world problems. Some commercial A.I. systems assemble dozens of different techniques. The results seem to keep improving so long as each part of the ensemble has a different technique from the others.