I understand the dictionary definition of 惣菜:
日常のおかず。副食物。
Basically, "a thing you frequently eat as an accompaniment to rice; a [non-staple] side dish".
Practically speaking, though, I don't quite get what kinds of things one would call a 惣菜. I have a sense for what kinds of things are おかず, but not what things are 日常のおかず, so some clarification about what sorts of things are considered 日常のおかず / 惣菜 would help.