When you open a link in a new tab with
target="_blank" like so:
<a target="_blank" href="http://some.link">Somewhere</a>
Then the page that you open has access to the original page through
Window.opener. Depending on your case you may not want this. Actually I can’t think of a case where you do want the new tab to refer back to the opening tab.
Window.opener access use
<a target="_blank" href="http://some.link" rel="noopener">Somelink</a>
At this moment, this is not available in IE or Edge, check out the caniuse site.
H/T Dillon HaferTweet