United States: if you are on OPT after graduating, keep a count of days without work: the total may not exceed 90 days, and for those with the STEM extension the combined total may not exceed 150 days

In plain words

OPT (Optional Practical Training) is the U.S. permission for F-1 students to do practical training and work after graduating, and during this period your status depends on having a job. If days without work add up to more than 90, your status may be terminated. For those who have the STEM extension, the combined total before and after may not exceed 150 days. An employer not registered in the school's system also counts as not having a job.