Inside this folder you must save mosquitto ssl certificates. Those are sensitive information! do not share. You can follow this guide to generate necessary certificates: https://mosquitto.org/man/mosquitto-tls-7.html or that one from Mr. Steve which i found more useful and complete http://www.steves-internet-guide.com/mosquitto-tls/. It is important to use broker with ssl to encrypt messages and to avoid someone bad intentioned to read messages exchange between the server and IoTs.