An XRef in 3ds Max is an externally referenced file or object. XRefs allow multiple animators and modelers to work on one scene at the same time without interfering with each other's work.
There are two ways to XRef another scene:
The file XRef Scene command XRefs an entire scene. for example, a classic use of this command is to bring in a scene that appears as a set for an animation you create.
The file XRef Objects command XRefs individual objects or materials. For example, you might be working on the model of a building, and fill it with furniture created by other artists.
You can also XRef the manipulators and modifiers associated with the objects you XRef.