This change means that popups won't block users from switching and closing the tab, putting an end to any overly-aggresive tactics on the part of the website's owner(s).
A similar change was made to Safari 9.1, released this week. Apple's decision came after crooks used a bug in Safari to block users on malicious pages using popups. Crooks then tried to extort payment, posing as ransomware.
In the meantime, Google is asking web developers to ponder using new technologies as alternatives to the classic alert(), confirm(), and dialog() functions. The list includes: