A water molecule has an uneven distribution of charge, this is because oxygen has a stronger attraction or pull on electrons than hydrogen. You can think of this like a game of tug of war, where oxygen is much stronger at pulling the electron in the bond closer to it than hydrogen. The tendency for an atom to attract shared electrons is known as electronegativity. As oxygen has a stronger attraction to shared electrons, it is more electronegative.

Cartoon depiction of electronegativity, a small white sphere representing hydrogen is holding onto a rope with one hand. At the other end of the rope is a red sphere representing oxygen, pulling on the rope with more force with two hands. Two blue circles, with a negative symbol on them, representing electrons  are attached to the rope and are closer to the oxygen atom. At the bottom of the image is a blue arrow pointing left to right, with a plus sign at the hydrogen end and a negative sign at the oxygen end.

What this means is that the charge distribution across the water molecule is uneven. Where water has a slightly negative end towards oxygen and a slightly positive end towards the hydrogen atoms, molecules that have an uneven distribution of electric charge are polar.