In addition to the other reply it means no longer putting in extra effort, and making the problems they face by following the rules managements problem.

For example, an employee might come to the office and find no open desks. They might have considered going back home and putting in a whole days work there, but work to rule could could mean alerting their supervisor that there are no available desks and ask how they should proceed. Refuse to work in conditions that don't meet ergonomic guidelines, take your time to set up work spaces to meet the guidelines if this is possible, spend the last bit of the day booking the next days in office locations, etc. Turn off your phone when the work day is over and don't look at your laptop after hours. If most of your day is spent dealing with hotelling, well that sure does suck - maybe they could make it easier for you to get your job done by removing these barriers?

If following the rules is a giant pain in the ass, you can make it your employers problem pretty fast. This works best if in a union and if organized as a union action.

It's basically group malicious compliance as job action. The employees find all the workplace rules that are on paper that no one actually fully follows ("drivers must check oil level before heading on a delivery") and then doing each and every one to its most obnoxious version (so a driver takes time to check the oil level between every delivery, even though they checked the oil already at the start of the day). As a result productivity suffers, and pressure is on management to concede something.