The phrase "to function at that level" means to perform at a particular standard or capability. It suggests being able to operate or work at the same level as something or someone else. Full definition
Conducted programs for residents on various functioning levels so that they can participate successfully in recreational activities. (resumemycareer.com)
With thought and preparation, you can keep insomnia at bay, and keep yourself high - function level for your nighttime job. (healthcentral.com)
A high blood sugar level is indicative of diabetes and low kidney functions levels will show if kidney disease is present. (wagwalking.com)