A chromatogram is the output of a chromatography run: a graph of detector signal against time, showing one or more peaks as different substances in the sample exit the column. Retention time is how long a substance takes to travel through the column and reach the detector — a consistent property of a given substance under the same method conditions, used to help identify what a peak represents. The relative area under each peak is generally used to calculate purity as a percentage of everything detected.
A purity percentage is a statement about chemical purity — how much of what was detected in the sample is the intended compound, relative to other detected substances — not a statement about how much material is physically in the vial. A 99% purity result does not, by itself, mean the vial contains 99% of its printed milligram amount; that is a separate question about fill content or assay, which is why identity, purity, and content are reported as distinct results rather than one combined figure.