An SSL (Secure Sockets Layer) certificate allows a website to use the HTTPS (Hyper Text Transfer Protocol Secure) protocol over the HTTP (Hyper Text Transfer Protocol) protocol. HTTPS provides a secure, encrypted link between the user's browser and the website, allowing for sensitive information such as bank details, to be transferred without the worry of the bank details potentially being intercepted and stolen by a hacker, like on the HTTP protocol.
Most sites now deal with sensitive information and subsequently have an SSL certificate. An easy way to identify if a site has an SSL Certificate is by looking at the URL of the site and looking for a green padlock.
However, SSL certificates can be falsified and that is where CAA (Certification Authority Authorization) records come in. CAA records allow the DNS record owner (the company that controls your domain name) to add a record which legitimises your own SSL certificate and proves that you own it, and the CAA record stops other people stealing your SSL certificate.