MetaTrader 4 Datacenters are proxy servers. They act as as point of entry to the primary MetaTrader 4 Live Trade Server. All MetaTrader 4 client terminals are connected to the primary server via one of the proxy servers.
Usually, the MetaTrader 4 Client Terminal will ping all the proxy servers and automatically select the nearest proxy server for connection.
At times, trader may want to manually connect to a specific less-favorable proxy server. This is made possible in MetaTrader 4 Desktop Client Terminal.
From your MetaTrader 4 Desktop Client Terminal,
click the connection status on the bottom left corner.
You will now see the list of available proxy servers (datacenters) and their response time.
The name of the proxy servers (datacenters) may be renamed from time to time, but generally it refers to the the codename of the server.
The list also indicates the response time (ping) in milliseconds.
Lower response time indicates a better connection.