When one substance is dissolved into another, a solution is formed. This is opposed to the situation when the compounds are insoluble and one of them precipitate like sand in water. In solution, all of the ingredients are uniformly distributed. No residue is left in the bottom. The mixing is referred to as miscibility, whereas the ability to dissolve one compound into another is known as solubility. However, in addition to mixing, both substances in the solution interact with each other. When something is dissolved, molecules of the solvent arrange themselves around molecules of the solute.
So, the answer to your question, "a substance that dissolves into another substance is what?", it is a solute. The substance that it dissolves into is the solvent. Together they make a solution.