It's a title given to long-time Apple employees who have distinguished themselves "in the service of Apple" - similar to Fellows in other companies. Generally, a Fellow is well-known in the industry as an expert/guru in one or more fields. It's a way to give high rank to employees that don't follow the management track (i.e., they can be a Fellow because they can't, say, be a V.P. if they don't manage anyone).