From the Smart TV Alliance website: Smart TV Alliance's open HTML-based solutions ensure its flexibility and allow partners to select their own preferred technology to create interactive experiences. The Smart TV Alliance SDK supports a simple yet great looking interface to give a perfect connected device experience.

JSTx is a command line utility to execute jointSPACE commands on a device that supports the jointSPACE protocol. JSTx runs on Linux PC's, Raspberry Pi's and Nokia N900 mobile phones.

Typical devices that support jointSPACE commands are Philips TV's from 2009 and 2010 (example type numbers of supporting devices 42pfl9664, 32pfl7605 etc.).
Refer to http://jointspace.sourceforge.net for details on jointSPACE and supporting devices. jointSPACE is based on DirectFB.
In essence, JSTx allows you to control and display graphics on top of your TV video, using a PC and a network connected to the jointSPACE enabled TV.