ARIA stands for "Accessible Rich Internet Applications".
Web accessibility refers to the inclusive practice of removing barriers that prevent interaction with, or access to websites, by people with disabilities.
aria-label is an HTML attribute that screen readers can read, so that visually challenged users can hear the content. This provides a mean for the challenged user to have similar experience as other users.
Similarly, aria-labelledby is another HTML attribute that takes a value as the ID of some other element and label the element with the content of the other element.
<div id="dLabel">This is the reference</div>
<ul class="dropdown-menu" role="menu" aria-labelledby="dLabel">
Therefore, when the screen reader read the ul element, it will read the content of the div with id="dLabel"