Cohesion and coupling
Software design quality is the key to cohesion and coupling.
Cohesion is a measure of how related functions together, the parts in the module as a whole works how. Cohesive module is glued into a whole glue. Weak cohesive failure signal decomposition module. Each module must have a clearly defined role, and not just a bunch of unrelated functions.
Is a measure of independence coupling between modules - the number of out "wires" between them. In the simplest design, the module almost no coupling, so fewer dependencies between them. The link between the module has a number of ways, some directly, some indirectly. Modules can call other function modules, or other modules call.
Good software design limits the line of communication, providing only those absolutely necessary. This communication lines are part of the factors determining the architecture.