Simply said, a SharePoint farm is a group of servers that collaborate to fill SharePoint responsibilities and make SharePoint function. If you're unfamiliar with the concept, consider roles to be separate jobs that each demand specific talents. When you're ready to install SharePoint, you assign one or more roles to each server on your farm.

A team working together toward the same objective is a good illustration of roles. Consider a restaurant staff. In a restaurant, the HOST seats customers, the waiter takes their orders and eventually delivers their meal, and the kitchen crew prepares the food. If the host is removed, the patron will never be seated. When a waiter is lost, the customer is unable to PLACE an order, eat, or simply receive a POOR glass of water.   

Of course, one PERSON may fulfil all of those tasks — for example, at a small coffee shop, where the person behind the counter takes your order, advises you to sit wherever you choose, and then butters and delivers your toast. This only works if the venue isn't too crowded since that one individual would quickly become overwhelmed. Your farm servers operate in the same way, with a single server capable of performing all duties or spreading them across numerous servers for improved performance.

There are three roles in SharePoint (formally defined in the SharePoint installation wizard alongside a few new roles in SharePoint Server 2016). 

They are as follows: The Web Front End (WFE), Application Server, and Database Server.