Absolute and relative pathnames are used in many different types of software.
I found a good explanation of the difference in the Help of ArcGIS Desktop (a specialty of mine) on a page titled Paths explained: Absolute, relative, UNC, and URL, but it should apply equally to Gramps:
Absolute, or full, path
An absolute, or full, path begins with a drive letter followed by a
colon, such as D:.
A relative path refers to a location that is relative to a current
directory. Relative paths make use of two special symbols, a dot (.)
and a double-dot (..), which translate into the current directory and
the parent directory. Double dots are used for moving up in the
hierarchy. A single dot represents the current directory itself.
The advantage of relative pathnames is that they promote portability. Let's say you have something located beneath C:\Gramps and you decide to move your Gramps folder to become D:\Gramps. If you used absolute pathnames then Gramps would not be able to locate it. If you used relative pathnames then it still would.
On the Gramps list a user says:
Enno has emphasized the benefits of the relative path to me both on
and off list and thus far, I agree with him completely. So much so,
that I'm wondering under what circumstances users would prefer the
absolute path? I'm sure that there is a very valid reason, but with my
own workflow I'm just not seeing it. Of course, it's always possible
that I'm doing something very wrong.
and the response from Enno was:
I can’t imagine having a preference for absolute paths, but I gather
that Gramps will use those anyway, if you try to add media outside the