Big Rock is a mental model which implies that to fill a bucket you have to fill the biggest rocks first and then progressively go on to smaller rocks.
Incidentally human brain also focusses on big rock heuristic. Unfortunately it sometimes captures the most vivid and emotionally compelling as the big rock in its analysis.
However in normal times- we attack the biggest problem first. That is a Big Rock mentality.
When does the Big Rock Mental Model fail? Have you heard of the saying – “the devil is in the details”. When there is a huge sensitivity in the system. So an arrangement where you have the big rocks inside the bucket but the little rocks when improperly put completely change the underlying assumptions.