The claim is that by strategically placing little round holes in the letters, you can use 25% less ink without sacrificing legibility. Ultimately, they want you to buy a plugin for MS Office that will substitute one of their fonts when you print something. (Only enterprise licenses are available at the moment.)
They offer one font for free, Vera Sans, so you can test the concept. I find the claim does stand up pretty well (in terms of legibility; I haven't attempted to measure ink usage). On screen and paper, the letters appear just a little lighter than normal, and the holes aren't that noticeable until you exceed 12 point.
Since it is intended only for evaluation, they don't offer bold or italic variants. Applications such as MS Word and some web browsers are able to improvise their own. For those that don't, I have created them using FontForge.
To learn more, visit Ecofont.eu.