The log is jut a reverse chronological order of comments/todos/rants.
I have mapped on VSCode cmd+shift+I to write a timestamp. Whenever I want to write something I just insert a timestamp at the beginning of the bullet-point list and write it out. This is stored alongside the repo.
It is particularly useful for recovering in the morning where I was the night before.
Like one of the top comments mentions, it acts as a node of knowledge within a wider system (graph) and I end up revisiting these logs more often than you would expect! It only gets better as you explore/document more and edges form.
I had no background in C++ or the GIS I was dealing with and this was a one-off thing that was built for a project, but not maintained or documented at all.
The lead eng that wrote the software also had a long and detailed personal log of what he was working on. He just gave me that.
It helped a lot to understand not only how the software worked, but also the design decisions.