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I’m no expert, but from http://www.alibris.co.uk/glossary/glossary-books#i “A set of copies of a work, printed at one time, from one setting of type. There may be several impressions of one edition. Also known as printing, press run, or print run.” (Strictly it refers to the number of copies in a press run, but it is usually used in its looser meaning.)
As I understand it there may be small corrections or changes between impressions but not as substantial as you would see in a new edition. And yes, first impressions, particularly of first editions, are more sort after among collectors and therefore more valuable.