This is a phrase used quite often by parents trying to teach children the difference between "can" and "may."
My mother chanted this like a mantra every time I mistakenly asked something akin to "Can I have a cookie?" or "Can I spend the night at Sarah's house?". The point being that yes, I am able to have a cookie, or make the trek to Sarah's house, but children with poor english skills are not allowed to have such treats as these. This of course traumatized me greatly, and directly influenced my choice of major.