Standard curves are used to determine the concentration of substances. First you perform an assay with various known concentrations of a substance you are trying to measure. The response might be optical density, luminescence, fluorescence, radioactivity or something else. Graph these data to make a standard curve - concentration on the X axis, and assay measurement on the Y axis.

Also perform the same assay with your unknown samples. You want to know the concentration of the substance in each of these unknown samples.

To analyze the data, fit a line or curve through the standards. For each unknown, read across the graph from the spot on the Y-axis that corresponds to the assay measurement of the unknown until you intersect the standard curve. Read down the graph until you intersect the X-axis. The concentration of substance in the unknown sample is the value on the X-axis.

In the example below, the unknown sample had 1208 counts per minute, so the concentration of the hormone is 0.236 micromolar.

Prism makes it very easy to fit your standard curve, and to read (interpolate) the concentration of unknown samples.