A certificate, in the context of web browsing, confirms the security and authenticity of a website. It's issued by a Certificate Authority, and a website's use of a certificate helps establish a secure connection via HTTPS. The certificate provides assurance that the web server is the entity it claims to be and that sensitive information is encrypted during transfer. A key is an item you typically keep private or share only with trusted parties, and a warranty provides assurance, often in terms of service or product reliability; therefore, these are not directly related to the role of a website's certificate.