1G1R stands for “One Game One ROM”. It is used for consoles rom and it means that you get only one file imported per game. Consoles dat often list a lot of regional versions of one game. Let's take an example. In the snes no-intro.org datafile, you get:
Super Mario Kart (Europe) Super Mario Kart (Japan) clone of "Super Mario Kart (Europe)" Super Mario Kart (USA) clone of "Super Mario Kart (Europe)"
There is only one game here, with 3 specifics regions versions. If you load that dat as-is in romcenter, you will get one main game (europe), and two clones.
Now you probably only want to see one version, the one for your region, and really don't care about the others. The solution is to enable the region filter before importing the datafile and give your regions preferences.
For example, set 'USA, Europe and Japan' in this order, and romcenter will only import the game with the best match. Only 'USA' version will be imported as 'main' super mario game.
Note that games not imported won't be known and identified by romcenter. They will appear with grey icon (unknown) and will be removed if you ask for a fix.
Not all datafiles are eligible to 1g1r feature, the 'region' data must be available with games.
Head to www.no-intro.org for compatible dat and more info.