Phones with TV output can be connected to a standard television for viewing photos, videos, and other content. Sometimes games and menus are also supported for viewing on the TV.
The connection is via cable, and a special adaptor cable for HDMI is required, which is not usually included with the phone.
Many phones that support this also support output to a computer monitor with a special adaptor cable for DisplayPort.
Some phones also support wireless streaming to a TV, which is a separate technology. There are several standards for this, including Google Cast, AirPlay, Miracast, and DLNA.