I found a definition for it in Solidworks help:
The SolidWorks Journal File is a text file that records the actions you
make in the current SolidWorks session. This file is often helpful when
you create macros.
Wayne Tiffany wrote:
It records some actions that the user makes in the current SW session
in a way that some users can turn into macros. It can almost never be
replayed as is to recreate what was done as some other software can do.
The primary reason for this is that SW does not open up the API so
that all possible user actions can be replicated with programming.
The eariliest version of SW did have the ability to recreate user
action, but this was abandoned very early on.