Certificate Authorities (CAs) are trusted entities that issue digital certificates, which are essential for establishing secure communications over the internet. These certificates verify the identity of websites and encrypt data exchanged between users and servers, ensuring privacy and security in online transactions. When a user visits a secure website, their browser checks the site's certificate against a list of trusted CAs. If the certificate is valid, a secure connection is established. This process is crucial for maintaining trust in online interactions, as it helps prevent fraud and data breaches.