It is generally a map or texture enhancement mod. The "plumping" part of the name usually refers to "filling out" the game world—adding more objects, vegetation, or structural detail to the Little Haiti area to make it look more modern or dense.
The .7z extension indicates it is a compressed archive that requires software like 7-Zip or WinRAR to open. Useful Review haiti-plumping.7z
As with any community-created .7z file from third-party sites: It is generally a map or texture enhancement mod
If you are looking to refresh the dated look of Vice City, this mod provides a noticeable boost to environmental density. It replaces empty lots with more realistic urban clutter. Useful Review As with any community-created
Because it adds more objects (polygons) to a specific area, players on very low-end hardware might see a slight dip in frame rates when entering Little Haiti, though modern PCs will handle it easily.