To host a website on the internet, an individual or company would need their own computer or server. As not all companies had the budget or expertise to do this, web hosting services began to offer to host users websites on their own servers, without the client needing to own the necessary infrastructure required to operate the website. The owners of the websites, also called webmasters would be able to create a website that would be hosted on the web hosting service's server and published to the web by the web hosting service.
You could think of the sites that share your server as your roommates; there's really not that much separating you from them. Sure, you can close the bedroom door, but they can still cause nightmares for you in the kitchen and the bathroom. In web hosting terms, all the sites share a single server's resources, so huge traffic spike on Site A may impact the neighboring sites' performances. It's even possible that another site could take down the shared server altogether, if it crashed hard enough.
Almost all the services offer some sort of page builder that makes it easy to drag and drop to build your page. These are great for getting started, but they often lock you into the service. Most page builders are proprietary to the service, or don't create HTML that's portable enough to be easily moved to another service if you decide it's necessary.