HTML - What does data-remote="true" do?

It is definitely not a standard thing.

Usually data-*** is a custom attribute used on application level. So check in sources of your scripts - it is used by some code.

data-remote = "true" is used by the Ruby On Rails framework to submit the form/link/button as an ajax request. If you are interested here is the guide discussing how Ruby on Rails works with javascript: http://edgeguides.rubyonrails.org/working_with_javascript_in_rails.html