There are two parts in dissolving - the solute (e.g. salt) and the solvent (e.g. water)
When the solute is added to the solvent, its particles are broken down, getting smaller and smaller
The solute's particles become so small that they can fit into the spaces between the solvent's particles, and they seem to disappear. This is the process of dissolving. The mixture that remains is called a solution.
Sometimes, dissolving does not always work. Some solutes will not dissolve in some solvents. These solutes are known as insoluble
Copper, for example is insoluble in water
The copper particles will not break down, and so the solution is cloudy, and you could even see the copper particles