A Reality Marble is the creation of an artificial world by enclosing reality, and then manipulating that enclosure to achieve a specific property or set of properties.
Essentially, Reality Marbles override the local reality of a region, like a boundary or "marble" if you will, and rewrite it according to certain parameters.
It can be said they are superficially similar to using Bounded Fields (areas with active magical properties) that can be cast with conventional magic, but the underlying mechanics are vastly different, which makes them superior to Bounded Fields. A Reality Marble does not just enforce a set of properties that reality then transcribes to "alterations caused by magic", it conditions the reality isolated inside the Marble to actualize and follow those properties like they were law. In that sense, the effects that can be achieved inside a Reality Marble are beyond the realm of magic.
Even to experienced mages, the casting of a Reality Marble is almost entirely unheard of. Theoretical knowledge about their construction and their implications certainly exists, but the execution is thought to be intractably complex to the point of practical impossibility, unless there is an outside factor involved. This "outside factor", of course, is almost always a Domain or a Soul Phantasm that is sufficiently powerful.