Polarity is when a molecule has a little bit of a charge on one side that's different from a charge on the other side so think of a north pole and a south pole when thinking of polarity.
So polarity is going to be a molecule that has different charges on the molecule so the bigger the difference in charge, the more polar. To make the definition easier, you can say that polarity is a difference in charge across a molecule.
Water is polar because it has a little bit of a negative charge on one end and a little bit of a positive charge on the other end so that's why water would be considered polar.