The concept is that a parking space is simply a small piece of urban real estate, available for (very) short-term rent at relatively low cost. The creators of this project decided that the usual assumption that these spaces are available to vehicles only is overrated. They fed the meter and created a 2-hour park -- complete with grass, tree and bench. Beyond pulling a creative prank, these artists were making a statement about how much of our public space is devoted to vehicles rather than people, and how much our physical environment affects what we do...as well as vice versa. Because people used this space, and enjoyed it.
The idea caught and spread, and a year later the same group led a Park(ing) Day, on which they "planted" five such temporary green spaces and other groups around San Francisco as well as elsewhere in the world joined in with many more. Their comment on the event:
In addition to calling attention to the need for broader discourse regarding public space in urban contexts, we sought to test public response to the PARKs in a variety of socioeconomic situations. We are pleased to report that the PARKs were generally met with a varying mix of surprise, approval, joy and incredulity. A few people thought perhaps we'd fallen out of our tree. Perhaps we have.I don't think so. I think they're brilliant.