What will happen if?

Salt is applied to raw mango pieces

The mango pieces will shrink due to osmotic pressure. The osmotic pressure of a solution is directly proportional to its concentration. As the concentration of salt (solute) inside the mango is less, the solvent starts flowing from the inside the mango to the salt solution outside to maintain equilibrium. So water inside the mango decreases to do exosmosis and will lead to plasmolysis. Hence the mango pieces will shrink and dry out.

In simple terms, pure water and salted water if kept separated by a semi permeable membrane, then water flows from pure side having high water potential to concentrated side having low water potential. Hence pieces will get salty and salt will act as a preservative as it will cause microorganisms to lose water by osmosis.