SSL stands for Secure Socket Layers, a security protocol. The protocol describes a process in which data transfer between the user and the server occurs in an encrypted form so that a third party cannot connect to the process.
SSL certificate affirms a clear connection between the owner of the domain name and the site that the domain name points to.
SSL certificate is obtained from various reliable providers; the reliability of the provider is confirmed by the historically established successful practice of using certificates of the current provider.
Each certificate has an expiration date, which means that you need to update it to have your site secure (usually is done once a year).