If something is modular, it is made of separate parts or units that you can put together, often in different combinations. Think of a sofa in sections, a kitchen of matching boxes, a school course made of optional units, or a machine whose pieces you can swap. The point is not only “it has parts”; the parts are meant to fit, come apart, and join again in a new way. Though actually, people stretch it: a modular home is built from factory sections; musicians talk about modular synthesizers. In higher maths you may later meet modular arithmetic, which is a specialist use of the same family — not this everyday sense.