The basic meaning of countersign, or countersignature, is when you have to write your name on a document that already has the name written of another person, especially in order to show that you are certain that the first person is who they say they are. Countersignature refers in that meaning to a second signature onto a document. For example, a contract or other official document signed by the representative of a company may be countersigned by their supervisor to verify the authority of the representative.
Furthermore, countersign is also common when talking about a money order or other financial instrument may be signed once upon receipt, then signed again by the same person when presented for payment, as an indication that the bearer is the same person who originally received the item, and not a thief who has stolen the item before it could be carried to the place where it was to be presented.
But in some cases, extra security is required when, for example, the transfer of particularly important documents is to be done. To increase security, there is a requirement for a third signature from a third party. link22’s product Countersign is for these very high-security cases, for example when a desired file transfer needs to be reviewed and approved by another person.