Best answer by dodge
It's telling you to stand up and do something else, go take a walk, be more active.
You can disable it on the Health app, just click on the watch icon (on the upper right corner), then scroll down and look at the options, You'll see it for sure.
Anyway, I wouldn't disable it, because it reminds you of how sedentary your lifestyle is.
Especially if you hear the buzz several times a day