Our speaker tends to get a bit metaphysical and otherworldly on us when he talks about time. Time, as we know, is a big part of our physical/earthly lives, or temporal lives, if you will. So to be "motionless in time" gives us the impression of being above and beyond all the physical stuff. We're not slaves to the clock anymore in "Ars Poetica."
Lines 9-10: A poem should be motionless in time "as the moon climbs." The speaker uses this line as a refrain later too. So we know that magical moon climbing effortlessly above us is a good simile for the timelessness of good poetry.
Lines 13-14: Good poetry also shouldn't be restricted to the temporal lives of any one individual's memories. It has to rise like the moon way above the physical and personal stuff.