I do not understand how stable Bitcoin you are while its value goes up and down in a short time
You're thinking of the *price*, not the *value*. Consider the technology of the wheel: while the price of wheels may vary drastically, depending on manufacturing location, roundness, shipping costs, tariffs, etc. the value of the wheel to humans doesn't really change.
Price is a highly imperfect mechanism that's used to *estimate* the value of a thing.