Activation subroutine is a set of procedures that have to be done so something would start happening. In Star Trek:VOY, the Doctor had activation subroutines. When one wanted to activate him, one only needed to say: "Computer, activate Emergency Medical Holographic program", at which point the Doctor would appear and asked about the nature of the medical emergency. That was the default mode. At first, he could only be activated (and deactivated) by any member of the crew, but not by himself. He once stated that he felt like a lightbulb because of this. That was later changed, so he could (de)activate himself when needed.