Instancing is a way to replicate scene geometry or meshes without using much additional memory - each instanced object shares the original mesh data, however the material and placement in the scene can be different.

This is useful in many situations where there are multiple copies of the same mesh data (such as chairs around a table, or trees in a forest) to reduce the memory usage required for detailing an environment.

An excellent example is given below by forum user Godzilla:

Image by Godzilla