What is website monitoring?
Website monitoring is typically an external service that checks on your website at regular intervals, logging the result.
What does that mean?
Essentially, a program visits your website every minute or so, checking if everything is still okay. If the website doesn't respond fast enough, the program lets you know.
Why should I monitor my website?
Monitoring your website via an external service lets you see how long your customers to load your website. Typically, the longer your website takes to load, the more your customers will leave before finishing what they came to your site to do.
Basically, if your website ever goes offline, you want a service like this to let you know - before your customers start telling you (often several hours later!)