The idea is to set up vacant tents on public property close to various financial institutions. The tents won't be "occupied" by a protestor but they'll be decorated with messages conveying the protest's ideals.
But "tenting" may actually be violating some laws, and can even amount to a crime in certain situations. Here's how:
Littering: Most states have littering laws on their books. Depending on the amount of litter you leave behind, you could either be fined or face a misdemeanor (or even a felony) in certain situations. This is because litter can cost cities by polluting sewers, draining financial sources, and generally just causing the city to look unattractive.
Trespassing: In most jurisdictions, it's also illegal to trespass, especially on private property. There's a bit more leeway on public property. But typically, you still can't go onto parts of public property that generally aren't open to the public. Depending on where you pitch the tent, you could be liable.
Obstruction of sidewalks: Many cities also make it illegal for individuals to completely obstruct sidewalks or block walkways. This is to ensure that pedestrians can get by. If you place a large tent on the sidewalk, most likely you'd be obstructing the passageway.
Disorderly conduct: You could also be cited for disorderly conduct if you are seen by police pitching tents on public property. Disorderly conduct charges can be brought against those who are disrupting the pace.
If you're thinking about joining the Occupy Wall street protestors by "tenting" in your city, you might want to read up on your local statutes to ensure you won't end up facing penalties.