Why do we see the stars only at night? Do they disappear in the daytime?

The stars do not disappear or turn themselves off during the daytime. They're just too distant to be visible on a light blue sky through the bright sunlight.

You actually begin to see the stars during the sunset, when the sky is still bright. The sundown is the time when the sky is not too bright to see the stars. But in the daytime the sky is lit by our powerful Sun, so the other stars cannot be seen on the blue shiny sky. It's just the same thing, when you barely see the text on your smartphone screen walking in the streets during a sunny day.