Scrutiny can send a regular request to your website (at whatever time interval you set). If the server's response varies from the expected response (usually 200 but you can find out what the expected response is for your page by using the 'test' button) then it will alert you by sending an email or popping up an alert.
It can also write any problems to a logfile. In the future there may be a feature to be able to view that logfile in the form of a graph. But at present it's a human-readable and machine-parsable text file. To prevent the logfile growing to a large size, it will only contain entries when the expected response wasn't received. Otherwise it's assumed that the request was sent and expected response received.
"varies from the expected response" means that your page / website is temporarily down or has some problem. Any website is unlikely to have 100% uptime (but usually close, 99.x%) and this tool will help you check that your server is performing consistently, and/or to give you an immediate alert if your website goes down.
With the current version, Scrutiny will need to be running on your mac in order for this feature to work. When a url is in the list and enabled, then Scrutiny will be monitoring that url.
Nothing will happen until the expected code isn't received.
Use the 'New' button to add a website to the monitoring list. Double-click an entry to edit it.
It's possible to specify a 'GET' or 'POST' request but at present it's not possible to add fields to a POST request body. If you're unsure what this means, then use GET.
The options found under 'Preferences' apply to all of the sites in your list. The email feature relies on the OSX Mail app being set up (but not necessarily running, it'll start up when needed).