(Link or link) This just means how one thing mixes with something else. Something can be soluble, so it mixes completely, like salt dissolving in water, or it can be insoluble, meaning the two things won’t mix, like oil and water. There are limits on solubility. Sodium chloride or table salt dissolves well in water, but you’ll reach a point where you have too much salt and no more can dissolve. The stuff that didn’t dissolve is called the precipitate.