What does "know the score" mean?

Also, know what's what. Understand what is happening; be familiar with the real story or the full situation. For example, It will take the new legislators some time to know the score, or When it comes to teaching youngsters to read, Nell knows what's what. The first expression, dating from about 1930, alludes to score as a tally of points in a game. The variant dates from about 1400.