Grafted cacti are composed of two plants growing as one. The colourful top plant is called Gymnocalycium Mihanovichii, which is a mutant Gymnocalycium lacking chlorophyll. This exposes its bright colours but makes it impossible for the plant to perform photosynthesis or feed itself. On its own, it would not survive.
Grafting solves this problem. It is grafted to a rootstock plant (usually Hylocereus species) with chlorophyll, and the two plants bond and grow as one.
Grafted cacti can be grown indoors, in a bright spot providing all year round colour.