A solution contains a solute dissolved in a solvent. A solid substance is soluble when its particles form attractions to the solvent particles that are stronger than the attractions between solute particles and stronger than the attractions between solvent particles. If the solute-solvent interactions are weaker than the solute-solute or solvent-solvent interactions the substance won't dissolve.
Here are some factors that increase the rate of dissolving:
Stirring or shaking: When a liquid containing soluble solid particles is stirred or shaken, the rate of collisions between the solute and solvent particles increases and new intermolecular attractions are established faster.
Temperature: Most solid salutes are more soluble at higher temperatures. This is because the particles move faster at higher temperatures, resulting in more frequent collisions between solute and solvent particles.
Particle size/surface area: A solid substance that is in the form of fine particles dissolves faster than large chunks because it has more surface area in contact with the solvent particles. For example, table salt dissolves faster in water than rock salt because the same mass of table salt has more surface area exposed to the water so more solvent and solute particles collide.