There are several S60 3rd phones already on the market now, for instance Nokia 3250, N71, N80 and N91. There will be some more phones in the near future. One of the new features of S60 3rd Edition is mandatory signing. It means S60 3rd applications, themes have to be signed otherwise they cannot be installed on S60 3rd phones.
What is signing? Basically application signing is a way of adding a tamper-proof digital signature to an application so that no one can modify the application. It will prevent the users from installing malicious or hacked applications.
The goal of signing process is good, but this has created some headaches to some people. One of the problems is installing self-signed application. What is self-signed application? It's an application that is signed with developer's certificate and key. Some applications use this kind of signing, for example SExplorer.
There is an option in S60 3rd phones to enable or disable self-signed applications. If you disable it, you will not be able to install self-signed applications. Some people choose to do it so that they can only install applications from 'trusted' sources. With so many Trojan viruses these days, it is sometimes wiser to disable this option.
If you enable it, you will be able to install any applications, including self-signed applications (but still you cannot install unsigned applications). Some people prefer to enable this option because it offers have greater flexibility. When you try to install self-signed application, you will get warning message, but you can still continue the installation.