It undoes the change made to `somefile` in `COMMIT_ID`. Details: `git show <COMMIT_ID>` gives you the patch identified by COMMIT_ID. Appending ` -- somefile` n

It undoes the change made to `somefile` in `COMMIT_ID`. Details: `git show <COMMIT_ID>` gives you the patch identified by COMMIT_ID. Appending ` -- somefile` narrows that patch down to only include the specified path. The `-R` option to `git apply` tells it to apply the patch in reverse. Thus, "undo the changes in `somefile` from this commit".