Our tooth is flushed with saliva day and night ,eventhough saliva is beneficial to the teeth as they are rich in calcium and many other substances , at times they tend to over accumulate calcium on the surface which gets easily mistaken as tooth only. As a result this tends to grow over time and starts to erode into gingiva shifting gingiva downwards negatively affecting the oral health by allowing bacteria to penetrate inside the gum. This can cause various negative impacts on the overall dental health. These deposits shows resistance to get removed with tooth brush and normal rinsing as they adhere strongly(sticky) to the tooth surface.
The purpose of scaling and polishing is to remove these deposits mechanically thus preventing oral cavity from further serious health issues.
Scaling uses specialized instruments to remove these deposits from tooth without harming them whereas Polishing is usually carried out after scaling to make your teeth feel smooth and shiny.