The Root certificate is the magic ingredient that allows Charles to decrypt HTTPS traffic. This certificate basically tells your MacBook to give Charles permission to view and modify all HTTPS traffic that flows in and out of your machine. This becomes incredibly useful when testing your application and you need to modify HTTPS traffic.
This article gives a good introduction to SSL/HTTPS proxies.
This tutorial will show you how to setup the Root certificate on you Mac so you can view decrypted network traffic from Chrome and Safari in Charles.
Download it from here
Download Root certificate
Open Charles -> Help -> SSL Proxying -> Install Charles Root Certificate
This should now open “Keychain Access” on your Mac
Open certificate in Keychain
The certificate should now appear (with a little red X to the left of it)
Double click to open the certificate. Expand the “Trust” section and select “Always Trust” from the first dropdown.
Apply Root certificate changes
When you click close the Keychain window you should be prompted to enter your admin password. Enter it and click “Update Settings”
Enable SSL Proxying
Open a browser -> go to any HTTPS enabled website you like -> open Charles and right click on a specific Request -> select “Enable SSL Proxying”
I am using NY Times in my example but you can test this with any HTTPS traffic
Check traffic appears in plain text
Reload the page on your browser. Now you should see Request and Response details in plain text