To understand dissolving, it is important to know what a solvent, solute and solution are.
During dissolving, solvent and solute particles collide. The solvent particles slowly move and surround the solute particles until the solute particles are evenly spread throughout the solvent.
An example of dissolving is when salt is stirred into water, as the salt dissolves in the water.
This means that in a saltwater solution, the salt is the solute and the water is the solvent. The solute and solvent are mixed completely.
There is a limit to how much solute will dissolve in a particular volume of solvent. When no more solute dissolves, it becomes a saturated solution.
Just because the solute dissolves does not mean it disappears. For example, if 1 g of salt dissolves in 50 g of water, then the mass of the saltwater solution is (1 + 50) = 51 g.