Run
The only thing on the scoreboard. Everything else in baseball is an argument about how to get one.
What it actually is. A run scores when a player touches first, second, third and home in order without being put out. That is the whole of it. The team with more runs when the game ends wins, and no run is worth more than any other: a solo home run and a runner scoring on a wild pitch are the same line on the scoreboard.
What somebody who knows the game gets out of it. Because every run counts the same, the interesting question is never how a run was scored but how likely a situation was to produce one. That is what separates a modern read of a game from a highlight reel. A double that scores nobody moved the odds more than a sacrifice fly that scored a man from third, because the double left a runner in scoring position and the fly ball spent an out. Runs are the outcome; run expectancy is the process, and over a season the process is what shows up in the standings.
The numbers worth knowing. A major league team scores roughly four and a half runs a game, and that number has moved a lot: under three and a half in 1908, over five in 1930 and again in 2000. A team that outscores its opponents by a hundred runs over a season is usually a playoff team, because about ten runs of difference is worth about one win.
What people get wrong about it. RBI is not a measure of how many runs a hitter created. It is a measure of how many men were on base when he hit, which is mostly a fact about the seven men in front of him. A hitter batting behind two men who get on base at a .400 clip will drive in more runs than a better hitter batting behind two who do not.
At Little League and travel ball. Youth games are much higher scoring, and not because the hitting is better. Walks, passed balls and errors move runners for free, so a youth inning can produce four runs without a ball leaving the infield. This is why run limits per inning exist at many levels, and why the most valuable youth skill is throwing strikes rather than hitting.
Batter's Eye: look up anything in baseball