Meaning of Github Ahead/Behind Metrics

Question

In plain language (hopefully with a simple example), what do the ahead/behind metrics on a Github repo\'s branch mean?

And what are the implications for that branch

Answer 1

The metrics like those you can see for this project describe, compare to a branch from the repo (like master):

• the number of new commits that the GitHub repo has done compared to another branch of another repo: those are the behind commits: the other repo is behind compared to the current repo (see those commits).
• the number of new commits another branch of another repo has done compared to the current repo: those are the ahead commits: the other repo is ahead compared to the current repo (see those commits).

The technical detail is illustrated by the script "determining which repos are ahead/behind origin":
It is about checking:

• what commits are reachable from another branch, but not from the local branch: ahead
  git rev-list "$localref..$anotherref"
• what commits are reachable from the local branch, but not from the other branch: behind
  git rev-list "$anotherref..$localref"