Tag Out
When the runner does not have to move, you have to go get him.
What it actually is. A tag out is a fielder touching a runner with the ball, or with the glove holding the ball, while that runner is not touching a base. It is required on any play that is not a force, on a steal, on a rundown, and on any runner who overruns a base other than first.
What somebody who knows the game gets out of it. The tag is where the difference between good and bad defenders at the receiving end shows up, and it is almost entirely about the catch first. A catcher or middle infielder who tries to catch and tag as one motion drops more balls than he saves time, which is why the modern coaching is catch it, then sweep. The slide rules at home and at second were written around this too: since the collision rule, a catcher may not block the plate without the ball, which turned the play at the plate from a physical contest back into a timing one.
The numbers worth knowing. A tag play at second on a stolen base attempt is decided by a few hundredths of a second, which is why replay review overturns so many of them and why pop time from the catcher is measured to two decimal places.
What people get wrong about it. A runner is out the moment he is tagged off the base, including after he has reached it safely and slid past it. That is why a man is safe at first if he overruns it and comes straight back, and out at second if he does the same thing, and it is the most common source of free outs at every level of baseball.
At Little League and travel ball. Sweep tags and holding the ball are the two things worth drilling, in that order. A youth infielder who tags with an empty glove or drops the ball on contact is giving away the play he just made, and a runner who slides past the bag at 10U is out roughly as often as one who never reached.
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