In game:
EDDI is a significant and customisable upgrade to your shipboard computer's personality sim, available at any starport and even out in the black via Subspace Torrented File Upgrade.
Out of game:
EDDI is a Windows desktop program that can run either standalone or as a VoiceAttack plug-in. Actions like jumping to a new system, buying and selling commodities, entering a signal source, scanning a celestial object, and many more will trigger voice responses. EDDI uses the journal log and the Frontier companion API to obtain information and track your actions.
When used as a VoiceAttack plug-in, the events that EDDI recognizes can be used to generate VA commands. For example, you can have VA automatically stow your landing gear when you undock.
EDDI also optionally integrates with third-party trade and exploration tools such as EDDN, EDSM and EDDB. Thus you both contribute data to these widely-used tools and benefit from the information that they provide.
Oh and yeah, EDDI can read GalNet articles aloud if you want. No need to wait for the standard-issue shipboard computer to receive a patch some time next year.