They are first editions/first impressions for that book in that format from that publisher. The presence of the 1 in the number line indicates this. If the 1 was missing and lowest number was a 2 then they would be first edition/second impressions.

They may not be "true" firsts i.e not the first time the book was published if for example they are paperbacks and the book was published in hardback first or if they were published previously by another publisher.