On hydrophobic surfaces, water runs off like pearls.
A dimension for hydrophobicity on smooth surfaces is for example the contact angle. The angle build by a liquid drop on the surface to this surface is called contact angle.
A surface is called hydrophobic when the contact angle is above 90°compare to water.

Measurement of a contact angle (Kontaktwinkel)

Waterdrops run down and take away dirt particles on inclined surfaces.

The contact surface for dirt particles is extremly reduced by the microstructure of the surface.

This surface is not hydrophobic and has no microstructure. It is more moistened by water, and dirt particles can adhere.