By default, when you browse an HTTPS website via Burp, the Proxy generates an SSL certificate for each host, signed by its own Certificate Authority (CA) certificate. This CA certificate is generated the first time Burp is run, and stored locally. To use Burp Proxy most effectively with HTTPS websites, you will need to install Burp's CA certificate as a trusted root in your browser.
Note: If you install a trusted root certificate in your browser, then an attacker who has the private key for that certificate may be able to man-in-the-middle your SSL connections without obvious detection, even when you are not using an intercepting proxy. To protect against this, Burp generates a unique CA certificate for each installation, and the private key for this certificate is stored on your computer, in a user-specific location. If untrusted people can read local data on your computer, you may not wish to install Burp's CA certificate.
For full instructions on installing Burp's CA certificate in your browser, please refer to the following article in the Burp Suite Support Center:
This article contains detailed steps for installing the CA certificate on various common browsers and mobile devices.