Here is list of things Function Keys on our keyboard usually do . There are other things they can do howver the list below is most used functions.
F1 :- Opens Help screen for almost every program .Windows Key + F1 would open the Microsoft Windows help and support center.
F2 :- Allows to rename a file or folder selected also allows to get into CMOS mode when tapped during boot.
F3 :- Opens a Search feature for the application that is currently active.
F4 :- Using Alt with this fucnction key (Alt + F4) closes active window.
F5 :- Refresh or reload the page.
F6 :- Moves the cursor to Address bar of the browser
F7 :- Mostly used for spell check in Ms Word.
F8 :- Used to access the boot menu in windows machines .
F9 :- Is shortcut to refresh sends and recieves emails in Microsoft Outlook, also refreshes a document in Microsoft Word.
F10 :- Usually opens a menu of the open application also when used with Alt (Alt + F10) it acts same as right click.
F11 :- Fullscreen Mode
F12 :- When used in Microsoft Word it opens save as dialog box.
Did you know Apple has F13 till 19 ???
Older IBM keyboard had till 24 ..