What "modular" means, in the phrase "Small Modular Reactor"

For small reactors the term "modular" can describe a single reactor that is assembled from factory-fabricated modules, where each module represents a portion of the finished plant. The use of modules implies that assembly has been reduced to limited activities such as connecting the modules, greatly reducing the amount of field work required, and simplifying completion. Taking this approach, this use of modules increases the ability to deploy a reactor in remote locations.

[Source: Argonne National Laboratory, "Small Modular Reactors", August 30, 2001]