Everything you should know about certificates and PKI but are too afraid to ask (smallstep.com)

With the recent eMudhra fiasco, it is important that people know and understand what "CAs/Certificates/Signatures/etc" are and how they come together to build a distributed system called "Public Key Infrastructure" (PKI) framework which has become the backbone of web security.

This is a longish article but well worth the time/effort and accessible to all.