Baseball, by the Rules
Disengagements
Once a runner reaches base, the pitcher has a couple of tools at their disposal to try to keep those runners from attempting to steal bases. These tools are collectively known as "disengagements," and come in two basic flavors: the step-off, and the pickoff throw.
A step-off is pretty much what it sounds like: the pitcher, who has come set, steps off the pitching rubber, possibly also faking a throw in the process. As a result, the runner retreats, at least temporarily deterred.
A pickoff throw is the pitcher throwing to an infielder, in the hopes that the fielder can tag the runner before they get back to the base1. If the fielder can get the tag in while the runner is off the base, the runner is out.
The important thing about disengagements is that a pitcher can only disengage twice2 in the same plate appearance. If they attempt a third disengagement, they have to catch the baserunner; if they don’t, it’s considered a “disengagement violation,” which results in a balk. Balks are discussed elsewhere, but for our purposes here just know that when a balk is called, all baserunners get to advance by one base; this includes baserunners who are not necessarily forced to move (for example, a runner on third with no runner on second). If the runner(s) end up advancing anyway, the number of disengagements resets; the pitcher can once again step/pick off twice before they need to get serious about it.