To start simply, a server is something which serves, it can be in the form of serving you via sending a message to your friend on WhatsApp or chatting with a colleague on Facebook, all this is done on a server. Servers are the backbone of internet and our lives. From making a banking transaction via some apps to playing a game, everything is done through servers. For example when you send a mail via Gmail you are working on the servers of Google which are making sure that your mail reaches to the recipient and also saves your mails and data for as long as you want. Now there are general servers like the servers of WhatsApp where multiple users are working and then there are dedicated servers which are used for specific purposes by a single user but using a dedicated servers has a cost and unlike WhatsApp it can't be used freely.
Now you also have heard of servers getting down or servers not working properly, that may be because of more number of users using or working at a same time which creates a load on the server and which in turn reduces the speed of the servers and it takes more time to load. Like many a times it happens that a banking website gets hanged and you are unable to carry out your transaction, it is because of the either too many users using that server at that time or a poor quality server. To combat this issues bigger companies always make sure that they use servers with high and top end configuration so that the work of their users is not hampered. Like it seldom happens that the website of Google gets hanged even when unprecedented number of users use them, that's because of servers used by Google are of top quality and of very good configuration which work smoothly even after rise in the number of users.
So, to understand it in short, servers are the reason you are able to send mails, order food online, chat on WhatsApp, surf on Facebook and everything which you are able to do on internet. In the end, I would just like to mention that servers are one of the most important inventions of the modern world and smooth functioning of servers is necessary for smooth functioning of our everyday activities and has become a need for mankind for survival.