Generally speaking "Modular(s)" refers to sets in the Creator Expert theme that are designed with common proportions and common attachment points to allow them to be strung together neatly. Mostly this takes the form of Technic-pin connectors and socket bricks in the same places on each set. It's made somewhat simple by all the sets being built on the same size baseplates, sometimes paired up the half-width version for larger sets.
They're also generally designed so that each floor of the building is itself a module. They have no cross-floor connections, allowing higher levels to be cleanly lifted off lower floors and separated to see what's inside.
While most of them are in the Creator Expert series, and that's generally what people mean when they talk about "Modulars" as a group, there are sets in other themes designed to a compatible specification. Off the top of my head:
There are some sets built with similar techniques (plate size, easily lifted floors) that are not directly/fully compatible with the other "Modulars": 71741 Ninjago City Gardens comes to mind. It doesn't have the sidewalks of the other sets, nor does it have all the attachment points.
Lastly... in the eyes of Lego themselves, "Modular" is not a theme. You can't shop explicitly for sets by their modular nature.
There's a few more themes than those listed above, but "Modular" isn't among them.