When we talk about solubility, we usually think of water as a solvent. However, water is not the only solvent and not everything dissolves in water. For example, nail varnish does not dissolve in water, but it in a liquid called propanone. As you may have guessed, propanone is used in nail polish removers! Other examples include gloss paint, which cannot dissolve in water, but in a solvent called white spirit. You can't wash off gloss paint, but you can remove it with white spirit.
We can explain what happens when something dissolves using ideas about particles. The particles in a solid are all held together in a fixed arrangement. When the solid dissolves the particles come away from each other, as they mix with the particles in the solvent. The diagram shows how the particles behave while dissolving.
Would you think a whole bag of sugar can dissolve in a glass of water? The answer is no! Even soluble substances like sugar cannot keep dissolving forever. A big bag of sugar is too much to be dissolved in a glass of water. You can find out how much sugar will dissolve if you add a little at a time. When no more sugar can dissolve, there will be some crystals left at the bottom. The solution is now saturated. It contains as much dissolved solid as it possibly can. Every solid has a specific solubility. For example, if 10 g of a solid can dissolve in 100 ml of water, we say that the solubility of that solid is 10 g per 100 ml of water.
Finally, temperature is another factor that affects solubility. More solid can dissolve in the same amount of water, when the water is hotter.