What An Edition Actually Means
Editioned work is not a reproduction, and the numbering carries specific information.

Prints and photographs are frequently sold in editions, and the terminology causes confusion because it resembles the language used for mass-produced reproductions while meaning something different.
What an edition is
An edition is a defined number of works produced from the same matrix — a plate, a screen, a negative or a digital file — under the artist's supervision, each considered an original work rather than a copy.
The distinction from a poster is that the edition is limited, authorised, and the matrix is conventionally retired or defaced afterwards so no further examples can be made.
Reading the numbering
A work marked as one number over another indicates its place in an edition of that total size. The number is generally not an indication of the order of printing and is not, in most media, an indication of quality.
Proofs
Beyond the numbered edition there are usually proofs. Artist's proofs are marked as such and traditionally retained by the artist. Printer's proofs and other categories may also exist.
The important question is how many proofs exist in total, because a small numbered edition accompanied by many proofs is a larger edition than the number suggests. A reputable gallery will tell you.
Size affects value
Smaller editions of comparable work generally carry higher prices, since scarcity is part of what is being sold. An edition in the hundreds is a different proposition from one in the tens.
Photography specifically
Photographic editions frequently vary by print size, with each size constituting its own edition. Establish the total across all sizes rather than only the one in front of you.
What to keep
The invoice, the certificate, and any documentation of the edition size including proofs. For editioned work this paperwork is a substantial part of what you own, and reconstructing it later is frequently impossible.