All the operations TheSign does bases on the existance of public and secret keys. Especially your secret key is very important, as he represents your digital identity. With loosing that key, you also loose your digital identity and you can't sign any document anymore (And least not with your old id, but to create and publish a new ID is a lot of work)
So it's essential to backup your keys and store them in a secure place. The „Backup Key“ function lets you select the folder where you want to backup your keys and copy all key data to it.
TheSign stores all files which came in by email and which you've signed in a default folder, which could be e.g. on a portable USB stick (depends where you've installed the program). If you want to store all these documents e.g. on a central server, this „Backup files“ functions helps you to copy all your files and signatures.
In opposite to a normal windows file copy operation, TheSign takes care that you do not accidentally overwrite some files and it also joins signature files together instead just overwriting them.
Image, you don't want to use your actual digital ID anymore or you can't, because you've lost your key data. For such a case GPG has foreseen the process of key revoke files. They work as normal public key files, but instead of adding a new public key to the other users keyring, they define an existing (means your) public key as revoked.
This process is irreversible, so think twice before revoke your key!
By choosing this function, your revoke key file is send to all users listed in your public key ring. When they copy this revoke key into their public key ring, your public key becomes revoked in their list.