Most stars cannot be seen during daylight hours because light from the sun is brighter than the relatively faint light from the other stars. This is largely a result of the Earth's atmosphere scattering the sun's light so that every point in the sky seems brighter than distant stars.
The atmosphere's role in scattering sunlight can be seen from Earth's orbit and the moon's surface. In these airless locations, the stars can be seen once the observer is shielded from both the direct light of the sun and the glare from the Earth. From the moon, stars can be seen even when the sun is at the peak of the sky as long as the observer is in the shade.