Modular origami consists of models (often, but not always, geometrical shapes) made out of many origami units, usually joined through folding tabs and slots, not glue or tape. In these examples (from Tomoko Fuse’s book “Unit Origami”, which I picked up at the Dogtown Book Shop) the units for each model are all identical. These are the kinds of folds that make good, colorful Christmas tree ornaments. If I do a class for Christmas tree origami, some simple modular origami will definitely be in the plan.
There are more complex designs that require two or more different kinds of modules for their construction. The units themselves are usually fairly simple to fold, but they often can be combined in many different ways to make everything from simple tetrahedra and cubes to fantastically complicated interwoven geometrical wonders. Try using your favorite internet search engine to look for images of “modular origami”, and prepare to be blown away.