Most chemical reactions occur in solutions. This is because a substance dissolved in a solvent, the solute, will be in its smallest state of subdivision, existing as individual molecules or ions that will increase their ability to react with other molecules or ions. Most chemistry in the body takes place in solution; in the absence of the solution, much of the chemistry of life would not take place. You are familiar with solutions that are liquid, like iced tea and sea water, but solutions can also be gases, like the atmosphere, or solids, like a gold ring, which is a mixture of silver dissolved in gold.
Solutions are mixtures composed of two or more substances in ratios that can change. Sugar dissolved in iced tea is a solution, but you can add more sugar if you like, and you still have a solution. By contrast, compounds are also composed of two or more substances (usually elements) but in a ratio that cannot vary. In water, there are 8 grams of oxygen for each gram of hydrogen. It won't be water if that ratio changes.