Machining features are areas that you define on a part. You may want to define an area where you want to use a specific type of operation or you may want to define an area to rework because there is too much residual material on the part.
Besides this, you can also decide to define a specific offset on an area of a part or a group of varied offsets on a part.
Useful information can also be found in the Using Geometrical Areas chapter.