What does pitching from the stretch means?

What does pitching from the stretch or from the windup means and what is the difference?
I often hear commentators say that a certain pitcher has miraculously changed his way from the stretch to a windup or vice versa?

In my opinion mlb pitchers never really change anything but im sure must be missing something?-)

Im not from the usa so the language and all the terms not come natural to me…tnx guys!

The difference is this: when a pitcher goes into the full windup (or semi-wibndup or no windup at all) he commits to delivering the pitch with no alteration or interruption. When pitching from the stretch, as most pitchers do with a runner or runners on base, he must come to a full stop on one second (usually with both hands at the belt) before delivering the pitch to the plate. Of course, there is nothing in the rules that says that a pitcher has to go from the stretch—a lot of pitchers are doing it all from the windup.
The problem with pitching from the windup all the time is that if a pitcher has a high leg kick or is slow to the plate the runner on first can get a good jump on him and have a better opportunity to steal a base. The use of the stretch tends to prevent this. But if a pitcher works fast and uses the full windup, or a slide-step instead of a leg kick, the runner has to think twice about making a break for the next bases if he doesn’t want to risk getting picked off! 8)