IMPORTRANGE is a function in Google Sheets that allows users to automatically copy data from one spreadsheet to another by entering the URL of the source spreadsheet and the range of cells they want to import, wrapped in quotation marks. To use this function, users must first open their destination sheet, select the cell where they want to import the data, enter `=IMPORTRANGE` followed by the source spreadsheet's URL and the desired range string, press Enter, and allow access if prompted. This function can be used to transfer data from one sheet to another without manual entry, making it a convenient tool for data management and automation in Google Sheets.