Common sense is optimization of decision making with heuristics and data compression. It comes at a cost-- cognitive biases, illusions, etc.-- but imagine trying to function in the world if you had to reason through and empirically test each decision.

"Will that lion eat me? How should I construct a study to AAAAAhhhh!!!"

In common use the term is also used to refer to intuition, which is something else. Intuition is also a bit of a sloppy term but mostly reveals to massively parallel so-called "right brain" modes of cognition that rely heavily on huge scale pattern recognition instead of linear paths of logic.