Developer A has a branch that he wants to merge with the master branch. He opens a pull request for that merge, and Developer B can look at the diff online and _comment on it_. On individual lines or blocks, or on the PR as a whole. The comments can become discussions, new commits can be added to the pull request and the comments are marked as outdated if that code was updated, when everything is OK developer B can press an "Approve" button and the branch can be merged.