A multi server environment is a term usually used when you have an Enterprise Server License, and have atleast 1 SQL-server, 1 SafeCom Master-server and 1 SafeCom Slave-server.
The Enterprise server license enabled the customer to have an unlimited number of slave servers. A slave server can help with load balancing, creating redundancy and reduce the bandwith usage to remote locations, among other things.
An environment without an Enteprise Server License can still use external SQL-server for the Master(standalone server in this case), device server can also run on a separate server, and multiple print servers with print queues, but they can not store the print jobs like slave servers can.
Some more information can also be found here:
https://help.bestofbreed.eu/when-do-i-need-a-safecom-multiserver-environment