The HTTPS monitor type checks for the availability of a web page secured by an SSL certificate. The monitor includes all of the checks and options you get with HTTP monitoring and in addition you get SSL certificate checking.
SSL certificate checks
Invalid and expired SSL certificate warnings turn away users. To prevent this situation turn on SSL certificate checks by activating Check SSL certificate errors on the Advanced tab of the HTTPS monitor.
Keep in mind that this is a simple check of the validity of an SSL certificate and its certificate chain. If you want more options, use an additional monitor of type SSL certificate to run further checks.
Setting up an HTTPS monitor
If you want to learn how to set up an HTTPS monitor, please have a look at the article Adding an HTTP(S) monitor for a quick how-to.