Evaluating bank_server:start () creates a global process named bank_server The local variable Data contains a data base of {Name, Balance} tuples which store the name and balance of the customer's account.
The bank server is accessed by sending it {from, Request} messages From is the Pid of the requesting process, this is included so that the bank process can send a reply to the requesting process (otherwise there would be no way of knowing who had sent the message)