What means just passing by in the McDonald's tv ad?

Answer 1

Rolf Harris.

I think it is adapted from night mail by W H Auden

Could this be a better fit?

Now the ladies of the harem of the court of King Catactacus, were just passing by.
All together, now the ladies of the harem of the court of King Catactacus, were just passing by.
Now the ladies of the harem of the court of King Catactacus, were just passing by.
Now the ladies of the harem of the court of King Catactacus, were just passing by.

Now the noses on the faces of the ladies of the harem of the court of King Catactacus, were just passing by.
All together, now the noses on the faces of the ladies of the harem of the court of King Catactacus, were just passing by.
Now the noses on the faces of the ladies of the harem of the court of King Catactacus, were just passing by.
Now the noses on the faces of the ladies of the harem of the court of King Catactacus, were just passing by.

Now the boys who put the powder on the noses on the faces of the ladies of the harem of the court of King Catactacus, were just passing by.
[Repeat 4 times]

Now the fascinating witches who put the scintilating stiches in the britches of the boys who put the powder on the noses on the faces of the ladies of the harem of the court of King Catactacus, were just passing by.
[Repeat 4 times]

Now if you want to take some pictures of the fascinating witches who put the scintilating stiches in the britches of the boys who put the powder on the noses on the faces of the ladies of the harem of the court of King Catactacus...

Answer 2

it means that you are just walking past the mcdonalds cafe and you've popped in