Periods are time and date ranges for the data displayed in the Phocas grid, and are often set to months or years. In Phocas a 'period' is often actually two date ranges: A current period and a comparative previous period. You can select a defined period, set a /wiki/spaces/userdoc/pages/589993 or, if you have permission, save a custom period.
The Period button automatically displays the default period for the selected database. For example, in the image below the button shows the default as 'Rolling 12 months'. Different modes will display different columns.