The scale and polish is essentially a deep clean of your mouth.
The dentist will use a special tool to get rid of the hard build up of tartar (calculus). Once the cleaning is done, she will then polish your teeth thoroughly to look nice and clean.
During the scale and polish process, the dentist will focus on some areas that you usually miss when you are brushing your teeth, in particular, the areas just below the gum line, behind your teeth and in the gaps in between them.
The scale and polish is one of the most important helps to fight against gum disease.