SNI stands for Server Name Indication. It allows the creation of multiple SSL certificates for the same IP address.
The problem with conventional use of SSL certificates was that a dedicated IP address had to be used for each certificate. The reason for this was that the encrypted connection was established before replacement of the name of the accessed web page, so the server did not know which SSL certificate it should send to the web browser. With multiple certificates on the same IP, errors sometimes occurred because the ‘first’ certificate was always used, which did not match all domains.
Today, it is possible to replace the name of the website before establishing the encrypted connection, thus providing the server with the correct certificate.
All common browsers now support this method. A few older mobile devices and Windows XP still sometimes have trouble with SSL certificates established by means of SNI.