Do somebody know any wifi tethering app on itunes

There can be no tethering apps. Tethering is built into iOS.

It means that is is part of iOS. It's already built into the phone.

http://support.apple.com/manuals/iphone/

"iOS" being the operating system that the iPhone uses. It is most likely built in to avoid apps on other platforms that allow you to tether, but only use the phones data (opposed to the optional tether data plan by your carrier).

cachavar wrote:

Sorry but I am new Iphone users, what does built into iOS mean?

See page 145 Here

http://manuals.info.apple.com/en_US/iphone_user_guide.pdf