That's just a general advisory from PayPal, not any specific issue with your own computer, PayPal account or shopping-cart integration. Most recent browsers and computers/devices from within the past decade or so should automatically have current root certificates.
As far as I can tell, this advisory really only affects super-techy sorts who manage servers, or people using really ancient computers that haven't been updated in forever. If you're using Windows 8.1 or newer (2013-onward), macOS 10.12 "Sierra" or newer (2016-onward), or a smartphone/tablet that's been released or updated in the last 5 years or so, you should be fine.
However, people still running Windows 7 or XP, macOS 10.11 "El Capitan" or earlier, or old smartphones/tablets that haven't been released/updated in the past 5 years or so, may need to update their root certificates manually, or else they may start getting error messages when they try browsing to secure Web sites.
To clarify, digital certificates are part of how your browser encrypts transmission of sensitive information like login/password entries, credit card details, etc. Root certificates are involved in that, as well as validating that a site is who they appear to be, rather than an impostor. Many folks refer to all this as "SSL" though that's technically an outdated term nowadays.
