While charging why the apple battery and charging pin getting hot

If your MagSafe cable or power adapter isn't working - Apple Support contains the following passage:

Check the power port for debris

Make sure that the power port (where you plug in the MagSafe connector) is clean and free of debris. 

An earlier version of that same document has a passage emphasizing the need for periodic cleaning along with some excellent illustrations. Any debris, including nearly invisible amounts of dust, will impede electrical conductivity resulting in the connection point becoming warm. That in turn places more demands on the power adapter so it also becomes warmer than usual.

Cleaning it is as simple as using a soft bristle brush, such as an old toothbrush. Don't use any solvents or cleaning products. Just brush it once in a while.

The next sentence regarding metal contaminants ought to be obvious on its face.

It means they are working. Refer to "If the adapter gets warm" in If your MagSafe cable or power adapter isn't working - Apple Support.

If you think something is wrong though, contact Apple. Bring your Mac, the power adapter, and cable — all three components — to an Apple Store or Apple Authorized Service Provider for evaluation.