SSL (Secure Sockets Layer) is a computer networking protocol that manages web server authentication, client authentication and encrypted communication between web servers and browsers.
A SSL Certificate contains a key pair - a public and a private. These keys work together in order to establish an encrypted connection.
If SSL is installed on your hosting account, your website can be accessed by both secure (https) and non-secure (http) connections. But the secure connection can slow down loading speed of your website. This is because the data transfer has be to encrypted/decrypted on both ends.
This is why a secure connection should be applied only on the pages that contain sensitive data, such as clients' personal information, credit card/payment details etc. And the rest pages of your website will be loaded faster with non-secure connection.