Two ingredients, one phase
A solution is a mixture so intimate that you cannot see the parts. Stir sugar into tea and it disappears — not because it vanished, but because individual sugar molecules have spread out evenly among the water molecules. We call the thing being dissolved the solute (usually present in smaller amount) and the thing doing the dissolving the solvent (usually the larger amount). The result is a single, uniform phase: one clear liquid, the same composition top to bottom.
This matters in pharmacy because a true solution is the most reliable way to deliver a measured dose. There are no particles to settle, no clumps to break up, no surface that has to dissolve in the body first. Every spoonful or millilitre carries the same amount of drug. That reliability is exactly why a syrup or an injection formulated as a solution is, in many ways, the gold standard — when the drug will cooperate.
Like dissolves like
Whether a solute will dissolve comes down to a swap of attractions. To dissolve, a solute molecule must break away from its neighbours, and the solvent must open a pocket for it; in return, new solute–solvent attractions form. If those new attractions roughly repay the cost of the ones broken, dissolving happens. Water is polar and rich in hydrogen bonds, so it embraces polar and ionic solutes (salts, sugars) and snubs oily, non-polar ones. Oil dissolves oil. This is the old rule “like dissolves like.”
When two liquids dissolve in each other in all proportions, we say they are fully miscible — water and ethanol, for example. When they refuse and split into layers, they are immiscible, like oil and water. Most drug molecules sit somewhere on a spectrum, partly at home in water and partly oily, and a big part of formulation is reading where a molecule sits and helping it along.
When the solvent is full
Keep adding solute and eventually the solvent stops accepting it. At that point the liquid holds the maximum amount it can at that temperature, and any extra solute simply settles to the bottom undissolved. This is a saturated solution. It is not static: molecules are still leaving the solid and rejoining it, but at equal rates, so the dissolved amount holds steady. That steady maximum concentration is the molecule's solubility — the headline number for the rest of this track.