This identifies the program and workbook you are working on. A workbook contains worksheets or spreadsheets. Each workbook can contain up to 255 spreadsheets. To the right you have three buttons:

Minimize If clicked, this button will shrink the window into an icon on the taskbar.

Maximize which has two states:
This button will maximize the window to the entire screen.
This button will shrink the window back to its original size.

Close If clicked this button will close Microsoft Excel.