The Batter's Box and the on Deck Circle
Where the hitter has to stand, where the next hitter waits, and the pitch clock rule that now governs both.
What it actually is. There is a batter's box on each side of home plate, four feet wide and six feet long, and the hitter must have both feet inside it when the pitch is delivered. The on deck circle is where the next hitter waits and takes his practice swings. The hitter after him is said to be in the hole, which is a corruption of in the hold.
What somebody who knows the game gets out of it. The box became a strategic object once the pitch clock arrived. A hitter now has to be in the box and alert with eight seconds left on the clock, which ended the routine of stepping out to adjust batting gloves after every pitch and took real minutes out of every game. It also shifted a small amount of control from hitter to pitcher, since the hitter can no longer break a pitcher's rhythm at will, and the one timeout per plate appearance he does get is now a resource worth spending carefully.
The numbers worth knowing. Four feet by six feet each. Under the pitch clock a hitter must be set with eight seconds remaining, and a violation is an automatic strike. Hitters get one step out per plate appearance.
What people get wrong about it. A hitter who steps completely out of the box with both feet while the pitcher is on the rubber can be charged an automatic strike, and a batted ball struck while a foot is entirely outside the box is an out. Both are called rarely and both are real.
At Little League and travel ball. The on deck circle is a genuine safety issue at youth level, because young hitters wander toward the plate to watch and get hit by thrown bats and foul balls. Most youth sanctions require the on deck hitter to be behind the plate line or forbid on deck swinging entirely at the youngest ages.
Batter's Eye: look up anything in baseball