Dissolving means that the solute diffuses into the solvent and mixes. It is a common case that salt or sugar is dissolved in water. Solvents are often liquid, mostly water.
The Earth’s atmosphere is a mixture of nitrogen and oxygen at a certain rate. Like this, there may be a dissolution phenomenon between substances in the same state too. In this case, the big amount is the solvent, and the small amount is the solute.
The cause of the dissolution phenomenon is the electrostatic attraction between particles and the increase of entropy.