I'm not exactly sure what a web host is. Can you please explain?

A web host is a company that hosts ot holds or provides space for your web site to be at.

Ok...

Any site that you view on the web is hosted somewhere.

In this sense imagine having your website hosted on your home computer or in that case on your laptop. Any time that you turn off your pc, blackout occurs, internet goes down, etc the site then becomes inaccessible.

Web hosts host your site on servers that are on 24*7 that are normally stored in data centres which have contracts with power companies and internet companies that stipulate the downtime allowed and they also have redundant systems. A good data centre will also have fire systems in place to minimise the risk of a fire or damage caused.