RUM stands for Real User Measurement and it is a technique for measuring the real world performance of websites. Unlike Synthetic monitoring, RUM captures the performance of actual end users in their many variations, ie device, browser, network speed and geographic location. By leveraging RUM, critical insights in site usage and geographic distribution can be gained by business to better understand its users and identify the areas requiring attention.
To gather measurements, a snippet of Javascript is injected/embedded in the target page. When the page is loaded in a client's browser the Javascript fires and uses the browser's event API to gather metrics about the loading of the page. It can then save/send back the information to analysing software where it can be collated and visualised.
The information gathered is comprehensive, ie full load waterfall and browser event timings, browser details and network connection speed. Since the information is from real visitors it shows the real world performance of a website and can be used to pinpoint issues/areas of improvement.
RUM is a passive measurement, ie it collects information without having any effect on the operation of the website.