Does anyone know what a "preview showing" is?

There was an article in the newspaper for a contest. What I have to do
is write an essay about Harry Potter, and the winner will get tickets
to a preview showing of "Harry Potter and the Deathly Hallows Part
2''. But what is a preview showing? Is it only part of the movie? Some
people said that you get to see the movie before it actually comes
out, but I'm not too sure. If anyone knows what a "preview showing of
the movie" is, can you please tell me? Thanks!!!!