When you publish an item in Sitecore through the "publish item" dialog, you are offered several options. You can choose between "smart publish" or "republish", you can choose whether to "publish subitems" or not.

These options are pretty self explanatory.

However, what exactly does the "publish related items" do?

[![Publish Related Items][1]][1]


  [1]: https://i.sstatic.net/NYJ3d.png