Web host is an organization/company which provides web space in their server for you to put your files and start your website.
For understanding purpose, lets assume you want to share certain text, files, pictures etc which you have in a particular folder in your local hard-disk (read computer). Of course you can access it easily on your computer but if you want me to access it, you will need to put on internet either on some else website or your own.
For placing those files on internet you will need servers which connect through internet and make these files available to the world. These servers are expensive things and demands proper maintenance and uptime which again demands more money. So you look for someone who manages these servers and are experts in this and prevent you from the pain of managing them yourself.
These experts are web host, they will provide you service as per your demand. Most big companies will either go for Virtual Private server(VPS) or Dedicated servers for their website with the web host. For small website owners the web host have options like free/cheap web hosting and shared web hosting. The web host will give you web space so that you can place the files there. I am using Shared hosting for my website and would never recommend going for cheap or free web hosting.
Now your files can be accessed by everyone by FTP (File transfer protocol) e.g. from your server to my computer but it is not secured. So you decided to start the website and others can find your files on website using HTTP (Hyper text transfer protocol) through a web browser like Internet Explorer e.g. from your shared server to my browser. But how will they know about the location of the website ? They will know and type it in the browser by the domain name of your website.
So, the web host will provide you a well maintained server space and register the website domain. These web space can be used for making the information / programs available to all through a website whose domain name too may be registered by the same web host.