"Relation Chain" is a combination of "Depends on" and "Dependent". All changes listed below the one you're looking at are ones it depends on, all changes listed above the one you're looking at are ones that depend on it.
"Submitted together" is closely related -- it's saying "if you clicked the Submit button on this change, which other changes would get submitted at the same time". It's slightly redundant for our case, but is necessary for many other Gerrit users that have different Submit strategies (e.g. ones that involve merges or cherry-picks).