Did you face any impermanent down time for the website you are monitoring?
This alert means that your website is having the string "error". This happens because you would have configured in the the url monitor or the webpage speed monitor the content check that the website should not contain the string "error". This is typically done so that when there is an error say 500 or other exceptions containing the word "error", it is captured and sent as alerts. This will help the Operations Engineer to look for the problem in the website proactively.