Purity (%)

When you receive a sample, your first question is: how much of it is actually the substance you ordered? That's exactly what purity states: what fraction of a sample is the desired substance - usually a percentage from HPLC (a common lab method that separates and measures the components of a sample), e.g. "99%". The rest are minor components.

What purity does NOT say

A high purity figure does not mean the right amount is in the vial (the small sample bottle): 98% purity of a mis-filled amount is still too little (or too much) active substance. Purity (what) and fill amount (how much) are two separate questions.

Analogy

Think of a juice bottle: purity tells you how much of the contents is actually juice (vs. water or pulp). Fill amount tells you how much is in the bottle at all. Both have to be right.