The Doctor blinked and rubbed at the corner of his eye. "Um. Not. Not a problem, exactly. Just... oh, don't worry, you'll find out soon enough."
He looked at Tosh to include both of them in his explanation. "You didn't get here on the TARDIS. You've slipped through to a pocket universe. It's... No-one knows exactly why we're here, or how we got here, but basically people arrive by accident and then suddenly find themselves back home again, at the exact instant they left and not remembering this place at all. It's sort of... out of time.
"And there are lots of different versions of me, and several other people, from different realities."
He smiled sheepishly, wondering if any of that would make any sense to them.