What you are asking isn't clear. Brogan has done his best to answer. He has explained that you use this button to make a hyperlink. He's also pointed you to the pages about BBCodes which explains how the BBCodes work to make a hyperlink.
I'll add something else:
Type some text, e.g. BBC website
Highlight that text.
Click on the Link button
Paste in the URL of the website you are linking to (in this case http://www.bbc.co.uk)