standard solution
A standard solution is a liquid whose concentration you know on purpose and with confidence — a reference you trust, like a ruler whose markings you are sure are correct. Everything else in the lab is compared against it. When you want to know how much of something is in an unknown sample, you measure it against solutions you have already pinned down.
Concretely, it is a solution prepared so that the amount of a particular substance in it is accurately known, usually expressed as a concentration such as moles per litre. You make it either by weighing out a pure, well-behaved chemical and dissolving it in a known volume, or by carefully checking an approximate solution against something already certified.
It matters because every quantitative measurement is really a comparison. The trustworthiness of your final answer can never be better than the trustworthiness of the standard you compared it to — if the standard's concentration is off by two percent, so is everything you measured with it.
Weighing 5.844 g of dried, pure sodium chloride and dissolving it to exactly 1.000 litre gives a 0.1000 mol/L standard solution of chloride ion.
Known mass + known volume = known concentration: a standard solution.
Not every chemical can be weighed straight into a reliable standard. Substances that absorb water from the air or react slowly are made up approximately, then standardized against a primary standard. Standards made this way are called secondary standards.