what is the definition of a primary standard?
A primary standard is substance that is available in a (1) pure, solid, soluble form, (2) such that it can be dissolved in water and made up to a solution, (3) the concentration of which is known to a high degree of accuracy.
As you can see it's one of the longer definitions you need for LC Chemistry but breaking it up will make it easier to learn. That would be the A1 textbook definition, but often the marking scheme would be slightly more forgiving.
Hope that helps