Learn French: J'ai vidé la poche parce que le billet avait glissé au fond. - I emptied the pocket because the ticket had slipped to the bottom.

The plus-que-parfait shows that a past action happened before another past action. Here, the ticket slipping to the bottom happened first, which is why you later emptied the pocket (J'ai vidé, which is in the passé composé). It translates perfectly to the English "had slipped".

I thought verbs of motion used être in compound tenses. Why does avait glissé use avoir?

It is true that many verbs showing a change of location or state (like tomber or aller) use être, but not all movement verbs do! Glisser (to slip or slide) is one of the verbs of motion that takes avoir.

Why is it au fond rather than à le fond?

In French, you can never say à le. Whenever the preposition à meets the masculine definite article le, they must combine into the contraction au. The phrase au fond is the standard way to say "to the bottom" or "at the bottom".

Does le billet always mean a ticket?

Not always! Un billet can mean a ticket for a train or a concert, but it is also the standard word for a bank note (paper money). Both meanings make perfect sense in the context of emptying a pocket.