There is a vulnerability in the web software for MacOS 10.14, 10.15 and MacOS 11 (Big Sur). More information is available here. Malicious actors could run code on a non-secured Mac.
Apple released security updates Monday March 8 for Safari and also for Big Sur. Please open System Preferences, Software Updates and click install to secure your Mac.
You can also copy and paste this into your browser window for more info:
https://macintouch.com/post/12468/apple-security-patches-4/