Photometry is used to determine a light curve. In essence, this is a curve plotting the light output of an object overtime. Each image will contain stars of a certain magnitude at that moment. A series of images are taken, the magnitudes are measured, and they are plotted over a period of time. The resultant variations of magnitude overtime produce a light curve, or a phase.
However, a single light curve does not determine, by itself, a period. This requires multiple light curves that together reveal a period, or a varying period as the case may be.
The reasons for magnitude variations, amplitude variations, and/or period variations gets to the heart of why we study photometry.
The following slides will outline examples of light curves