It's a way of being in agreement and saying yes; it also means your actions or decisions in any given situation are predicated on the actions and decisions of another; you march to the beat of somone else's drum; you're willing to go with the flow; You're ready to do whatever they want or ask of you; ready to do things on their terms.
"Do you want to come with me to the mall, or are you busy?"
"I'm on your schedule, Madame."
"Since you're off tomorrow, could you drive me to work? I have to be there at 8am."
"I'm on your schedule."