Clickjacking, also known as a "UI redress attack", is when an attacker uses multiple transparent or opaque layers to trick a user into clicking on a button or link on another page when they were intending to click on the the top level page. Thus, the attacker is "hijacking" clicks meant for their page and routing them to another page, most likely owned by another application, domain, or both.
Using a similar technique, keystrokes can also be hijacked. With a carefully crafted combination of stylesheets, iframes, and text boxes, a user can be led to believe they are typing in the password to their email or bank account, but are instead typing into an invisible frame controlled by the attacker.
Clickjacking at a glance means, that someone could "steal" your page (or parts of it) and make it look like it's his own content.
There are three possible values for X-Frame-Options:
The page cannot be displayed in a frame, regardless of the site attempting to do so.
The page can only be displayed in a frame on the same origin as the page itself.
The page can only be displayed in a frame on the specified origin.
Header always append X-Frame-Options SAMEORIGIN
... <httpProtocol> <customHeaders> <add name="X-Frame-Options" value="SAMEORIGIN" /> </customHeaders> </httpProtocol> ... </system.webServer>
add_header X-Frame-Options SAMEORIGIN;