The term "shared hosting" refers to the usage of a single webserver to host multiple websites for multiple customers. It is generally used in comparison to "dedicated hosting" or "dedicated server hosting" where you typically have a single webserver dedicated to hosting a single website (or multiple sites for a single customer).
In the beginning of "the web", when you needed a website, you set up a server with webserver software, and built your site on it. But as more folks started adding small websites with small amounts of traffic, it quickly became clear that you didn't need a whole server just for one website, that in fact, 1 physical server could be able to host a large number of small websites... those websites could "share that server's resources".
Today, the two main web server solutions, Apache for Linux, and IIS for Windows, both provide the ability to host multiple websites on a single server, all sharing a single IP address and other server resources (CPU, memory, etc). Each provides an implementation that isolates each site's code and user priveledges to ensure approriate security, although each takes a slightly different approach.
So, in short, shared hosting means you have many websites on a single server.
One separate note on SSL certificates... you can host your own unique SSL certificate on your website on a "shared hosting" solution. However, you need to not use the "shared IP address" that all sites on a shared server use by default. Instead, you'd need your site configured to use it's own unique IP (this is due to the nature of how the server handles HTTPS/SSL communication). Check our website hosting plan pages for prices for adding a unique/static IP to your hosting plan as well as SSL certificates, and then submit a ticket if you'd like to make this change.
3Essentials also offers dedicated server plans. A dedicated server is a server dedicated to a single customer... that customer may chose to host 1 big site on the server (that needs lots of CPU and memory), or may host many websites on it, depending on their needs.