This question from The Edge and the dozens of thoughtful answers make for some good reading. Basically, each author picks a single idea they feel is necessary for everyone to “get” in order to understand the world we live in; to have a successful technological civilization.
I found this via Pharyngula, who suggested that the Mediocrity Principle may be The One. That is, the basic understanding that we are not the special reason for the existence of the universe. His argument is that basic math skills would help. We’re talking about skills that even average college students seem to lack, but are nominally taught to most people who graduate secondary schools.
Adjacent to PZ, Sue Blackmore argues for the primacy of understanding that CINAC (Correlation Is Not A Cause). Apparently this lesson is hard to drum into even college students who are nominally studying science.
Most of the answers are direct explanations of ideas necessary to scientific understanding. But a few are more of the “what would be nice to discover” variety. But go see for yourself. There are many insightful replies to this question by 160 authors.