Learn Polish: Restauracja jest za dworcem. - The restaurant is behind the station.

The preposition za means behind here. When talking about a fixed, stationary location where something is, za requires the next word to be in the Instrumental case. For a masculine noun, the Instrumental ending is usually -em.

I notice the 'e' from dworzec disappeared in dworcem. Why did that happen?

This is a common pattern in Polish called the fleeting 'e'. Many masculine nouns ending in -ec or -ek drop that final e when you add a case ending. This makes the word easier to pronounce. So dworzec plus the ending -em shrinks down to dworcem.

Could I just say za dworzec instead?

Not in this sentence. If you use the basic form or Accusative case and say za dworzec, it implies motion toward a destination (like walking to the space behind the station). Because the restaurant is just sitting there in a static location, you must use the Instrumental case dworcem.

Does za always mean behind?

In the context of physical space, yes. However, you will also see za used to talk about time, such as za godzinę meaning 'in an hour'. When used for time, it uses a different grammatical case, but for physical locations it means behind.