Extensions are a means to extend maxwell capabilities in several areas: geometry loaders, procedural geometry, scene modifiers and procedural textures.
Geometry loaders allow geometry to be loaded at the beginning of the render process. It's an external referencing mechanism that substantially decreases the size of .mxs files.
Procedural geometry extensions enable maxwell to render mathematically defined primitives. It is geometry created on-the-fly. Given a ray, with origin and direction, their main task is to calculate an intersection point and a surface normal. In addition to this, given an intersection point these extensions can also calculate UV coordinates, used mainly for texturing purposes. An extension can define an arbitrary number of ways of generating these coordinates.
Scene modifiers are able to change elements in the scene just before the render starts.
Procedural textures generate texture colors on-the-fly while rendering.