Instancing is a feature which allows many instances (or copies) of a mesh to be represented in the scene, while only actually storing the mesh once (thereby saving a lot of memory compared to straight-forward duplication).
There are two ways this can be achieved in SkIndigo:
A proxy is a reference object that references a more complex object with a usually simpler copy, in order to keep the overall scene geometry low, while allowing the render geometry to stay high.
Copies of components in SketchUp will export using Indigo instances.
Component copies will be exported as Indigo instances - each component copy will only use a small amount of memory.