OK - a friend explained it to me this way...

for example - the background microwave radiation which originated shortly after the big bang.

- Take a mirror and reflect that radiation directly back the way it came.
- hitch a ride on :)
- go to sleep and set yr alarm for 13.7 billion years.
- when you wake up you should be back exactly at the rediation's original starting point, but 27.4 bn years after the event.

this is the interesting bit - as the background radiation from the big-bang is observable in the sky no matter which direction you look.

- forget the first example, and imagine doing the experiment again. But this time you have a friend with another mirror reflecting the background radiation that's coming from the opposite direction to yours.
- now you're both hitching rides on the backs of your individual radiation, but in totally opposite directions.
- in 13.7 billion years both of you arrive at the radiation's starting point - and here is where you and your friend meet again.
- this is like if you and your friend both started out in opposite directions starting from the north pole of the earth, and meet again at the south pole. You've both travelled around the edge of a globe - or a sphere.

This works nicely for me :)