A solution is a mixture of materials, one of which is usually a fluid. A fluid is a material that flows, such as a liquid or a gas. The fluid of a solution is usually the solvent. The material other than the solvent is the solute. We say that we dissolve the solute into the solvent.
Some solutions are so common to us that we give them a unique name. A solution of water and sugar is called syrup. A solution of sodium chloride (common table salt) in water is called brine. A sterilized specific concentration (0.15 molar) of sodium chloride in water is called saline. A solution of carbon dioxide in water is called seltzer, and a solution of ammonia gas in water is called ammonia water.
A solution is said to be dilute if there is less of the solute. The process of adding more solvent to a solution or removing some of the solute is called diluting. A solution is said to be concentrated if it has more solute. The process of adding more solute or removing some of the solvent is called concentrating. The concentration of a solution is some measurement of how much solute there is in the solution.
It might initially offend your sensibilities to consider a solution in which the solvent is a gas or a solid. The molecules of a gas do not have much interaction among them, and so do not participate to a large extent in the dissolving process. Solids are difficult to consider as solvents because there is a lack of motion of the particles of a solid relative to each other. There are, however, some good reasons to view some mixtures of these types as solutions. The molecules of a gas do knock against each other, and the motion of a gas can assist in vaporizing material from a liquid or solid state. The fan in a 'frost free' home freezer moves air around inside the freezer to sublimate any exposed ice directly into water vapor, a process clearly akin to dissolving. Solid metals can absorb hydrogen gas in a mixing process in which the metal clearly