I'm not sure that video would actually be more useful than words here, particularly since I have no way to record a voiceover for a screen capture. The docs that describe the substring function are here:
...but I'll recap here:
These are all optional settings.
"Substring required" is a case-sensitive pattern to search for in the web content retrieved by our availability monitor. When set, our availability monitor will look for the case-sensitive series of characters you've entered in the web content we receive, and consider your site to be down in case that exact string is not within the first approximately 40k of the content from your application. An example of when to use this is if you expect www.yourcompany.com to have your company name on it, so you put in "My Company Name" since that is in the HTML of the page. Then, in case one of your employees removes that text, or some portion of your technology fails slightly so that the webpage we're monitoring no longer says that, you'll get alerted as though your site were unreachable.
Normally our availability monitor does not consider an HTTP 3xx (eg: 301) response (also known as a redirect) as a "successful" page load and alerts you if your site is not returning an HTTP 200 response (which is the typical response for a successful page load). If you are monitoring, say "http://yourcompany.com" but the webserver instead says "hi browser, automatically instead load http://www.yourcompany.com", we'll consider that a failure - unless this box is checked. Note that enabling redirects means that we will NOT search for any substring you've specified. We just see an HTTP 3xx and mark the load as successful.
Finally, if your site has an HTTPS url (eg: https://yourcompany.com), we can perform basic SSL validation - that is, we should notice if your SSL certificate is not working right. In a web browser, these kinds of errors frequently manifest as pop-ups advising the visitor that a site is insecure or maybe malicious. In practice, during our monitoring, these errors tend to pop up most often when your SSL certificate expires, or is misconfigured - both things that make you look bad to your customers.