The SAP MaxDB X servers are the communication servers of the SAP MaxDB database system. They are required to connect to databases on remote computers. Note that the JDBC interface, and therefore Database Studio, requires an X server even for local communication.
To establish a connection to a database on a remote computer, applications (via interfaces) and database tools use an X server running on the remote computer. There is one X server acting as a global listener per database computer, and one X server per SAP MaxDB installation. This means that on each database computer, there are at least one global listener and one X server.
On Unix and Linux, the X server processes are called vserver.
SAP MaxDB Security Guide, Starting SAP MaxDB X Servers Without NI Support (Unix and Linux), Accessing Databases Behind a Firewall
Database Administration in CCMS: SAP MaxDB, Remote SQL Server