A movement built by adding a functional module, such as a chronograph, on top of a separate base calibre. Contrasted with an integrated movement, designed as a single unit from the outset.
A modular movement takes an existing base calibre, often a simple time-only movement, and adds a self-contained module on top to provide an additional function like a chronograph or calendar. It's a faster, cheaper way to bring a complication to market, since the base movement doesn't need to be redesigned.
An integrated movement, by contrast, is engineered from scratch with the complication built directly into the base plate and bridges, rather than stacked on top. Integrated chronographs are typically thinner and often regarded as more technically sophisticated, since every component is designed to work together from the start, but they're significantly more expensive to develop.
Not necessarily lower quality, but integrated movements are generally considered more technically sophisticated and are typically thinner, since they're engineered as a single unit rather than built by stacking a module onto an existing base.
It's faster and considerably cheaper to develop, since an existing base calibre can be reused rather than designing an entirely new movement from scratch.
Often, yes, modular chronograph movements tend to be noticeably thicker than integrated ones, since the module sits stacked on top of the base movement rather than sharing its plates and bridges.