General Http check of your website. Each test simulates downloading of the page by a real customer. For 24x7 monitoring you should register, in case of a problem you will receive instant notification by e-mail, SMS, Hangouts, Skype, Slack, Telegram or phone call.
For those seeking a possibility to keep SSL certificates always up-to-date and have a constant watch over their expiry dates, there is a Certificate expiration function. Read more: How to activate a Certificate expiration function?