As seen in the pictures, it removes the frame and background from the small minimap.

The original Map without borders mod was designed for Rebirth, and if you don't have Afterbirth you should use that one: http://moddingofisaac.com/mod/573/mapwithoutborders

This mod is for Afterbirth, since the new minimap1 file is different. You should use this one instead of the old one if you have the Afterbirth DLC.

EDIT:
The last patch broke all mods. I'm working on a way to update that, but I can't rely on any method since they can change everything the next day.

Currently I've found a work-around for all old mods:

1. Back up your mods in the "resource" folder or copy them somewhere else temporarily.
2. Use the "Resource Extractor" in the "tools" folder. This will extract all ".a" files into the "resource" folder.
2. Rename the "packed" folder into something else, like "old_packed". Now the game will not find the "packed" folder and will launch the game using the extracted files.
3. Place your mods back into the "resource" folder.

This is only a temporary solution, but it works for all old mods.

Steam Workshop edition: http://steamcommunity.com/sharedfiles/filedetails/?id=844939659