Spreadsheets, also called worksheets, allow for the creation and organization of tables and data. Most spreadsheet editing is now performed using computer software, such as Microsoft Excel or Apple Numbers. While you could feasibly create a spreadsheet for anything, there are certain types of spreadsheets commonly used to provide such versatility.
An active worksheet is the worksheet that is currently open. For example, in the picture above, the sheet tabs at the bottom of the window show "Sheet1", "Sheet2", and "Sheet3", with Sheet1 being the active worksheet. The active tab usually has a white background behind the tab name.