What is a standard solution?
A solution whose concentration is known to a great degree of precision and correctness.
In analytical chemistry, all of the methods envolved - except gravimetry - require either a solution of known concentration either for a direct reaction (titrometric methods) or for a calibration curve (other methods), so we use standard solutions.
They may be primary standards - i.e.: solutions of salts that can be obtained in extremely high purity, such as
And they may be either made "in-house" so to speak or be bought from another lab - generally with a certificate of purity.