Web browsers Google Chrome (with 57.69% global market share as of March 2018) and Firebox (with 5.4% global market share as of March 2018) will start to distrust all the websites with SSL/TLS certificate issued by Symantec, Verisign, Thawte, GeoTrust and RapidSSL.
This means that soon in the near future, every time when you visit such websites using HTTPS protocol with Google Chrome, Firebox and possibly other web browsers which follow suit, the browser will give you a security warning before you can read their webpage.
Some of the affected popular websites including (but not limited to)...
In late 2017, DigiCert has acquired Symantec's Website Security and related PKI solutions which was the Certificate Authority for those affected Symantec, Verisign, Thawte, GeoTrust and RapidSSL SSL/TLS certificates.
Webmasters of all the affected websites can make arrangement with DigiCert to replace their SSL/TLS certificates with a new one issued by DigiCert, which is still trusted by Google Chrome and Firefox.
You can click here to read for more information about this issue.