The atmosphere of our Earth is a colloidal mixture of dust particle, smoke particles, molecules of air, water droplets, etc. Some particles are so small that when light is incident on them, it gets scattered instead of getting reflected.
Smaller particles scatter light of shorter wavelength such as blue colour. During the day, when light from sun enters the atmosphere, the blue coloured light gets scattered by minute air molecules in all the directions. (Of course, the light with longer wavelength such as red colour gets scattered but in very less proportion as compared to blue light.). Thus blue coloured light gets scattered all around the sky. When this scattered (blue coloured) light enters our eye, we see the sky to be blue coloured.