What is the use of a web-server on the internet?

A webserver is a computer that has been set up to listen for incoming requests from a client, find appropriate data for that request and return a response to that client. This is similar to a 'chatbot' on Facebook Messenger, if you want to order a pizza using the Dominos chatbot, you have to send a message saying 'Can I have a pepperoni pizza?', the bot will receive this request, notice that you asked for a pepperoni pizza and send a response like 'Of course! What size would you like?'. This interaction is very similar to a web-servers job on the internet. A client (that's you using your web browser) will send a HTTP request to the web server asking for a specific file by calling a URL (i.e. if you want the maps application from Google you'd type in www.google.com/maps). The web server will then recieve this request, check if theres any data contained within this request (i.e. you want the maps application) and return an appropriate response which is usually a web page (i.e. the maps application). Have a go in your browser and see what Google returns when you type in www.google.com/mail.