This is a certificate deployed by default with GroupShare installation and it is used for internal communication. It serves as a security layer for the WCF endpoints as this needs a certificate to be encrypted, deployed by default with a GroupShare installation.
This certificate gets renewed every time an update to a higher CU or upgrade to a higher version of GroupShare is initiated on the Application Server.

Note:
In situations where no updates/upgrades are installed, is normal for this certificate to expire after a period of time (12 months or more) but there is a way to renewing it. On the GroupShare Application Server you can run the following tool with Administrator rights: C:\ProgramData\Package Cache\SDL\SDLTradosGroupShareversion\CertificateRefresh.exe