When the client and server use WS-SecureConversation (by setting authenticationMode to "SecureConversation") the client get some token from the server in the initial handshake and needs to send it in every connection attempt. The server can extract this token and find the relevant data of this session in its memory. But what happens when the server/IIS is restarted? The memory is cleared and the session becomes invalid. The way to overcome this is to set requireSecurityContextCancellation to false (the default is true). In this mode the session key is saved at the client side and it sends it to the server in every request. This means that even if the server is restarted the session key is available in the request. This also means that the size of each request is a little bigger.
Since with this mode the server does not need to save the session key it does not need to be informed when the session is over and from here the property name requireSecurityContextCancellation.
BTW From performance point of view it is not always recommended to use a secure conversation.
Read more on the subject in the MSDN page.