The introduction to this article provides insufficient context for those unfamiliar with the subject matter. Please help Wikipedia by improving the introduction.
Characterized by:
Functional partitioning into discrete scalable, reusable modules consisting of isolated, self-contained functional elements
Rigorous use of welldefined modular interfaces, including object-oriented descriptions of module functionality
Ease of change to achieve technology transparency and, to the extent possible, make use of industry standards for key interfaces.
The "modules" I'm referring to are design elements you develop for using in a particular project---the specific headings, rules, buttons, and other paraphernalia united by the general style of the site, plus the abstract aspects of your composition: fonts, colors, textures, alignment styles, etc.
Thus, "modulardesign" is just another name for one of the most important design principles, consistency, which states that you should strive to minimize the inventory of different elements and materials used in any single composition.
In fact, this "pure" design example could be especially instructive because you won't be distracted by the text (in English design books, Latin text is often used with the same purpose).
Because modular units are generally focus on solving a single problem and work as part of a wider network but are given independence within the network, they tend to be much more flexible in terms of adapting to changing market conditions.
For example, a modular by design company could be relying on a number of other companies to help it bring its product to market, and could even be working with its competitors in order to push forward a new idea, standard, or product line.
Amorphous by nature, the modular by design approach means that a module has already moved on by the time someone has enough information to pin it down.