Older 3GP videos often looked "stretched" or pixelated. A "fixed" version usually means the video has been re-rendered to look correct on modern 16:9 smartphone screens.
To understand the "3GP" part of the query, we have to look back at the early 2000s. 3GP is a multimedia container format defined by the Third Generation Partnership Project (3GPP). It was designed specifically for 3G mobile phones to save disk space and bandwidth.