The mod_ssl module adds an important feature to the Apache2 server - the ability to encrypt communications. Thus, when your browser is communicating using SSL encryption, the https:// prefix is used at the beginning of the Uniform Resource Locator (URL) in the browser navigation bar.
The mod_ssl module is available in apache2-common package. If you have installed this package, you can run the following command from a terminal prompt to enable the mod_ssl module:
sudo a2enmod ssl