If you are a normal person, 'urbanite' usually means someone who lives in the city. It is a conjunction of urban + -ite. It may imply a high level of yuppiness.
But on the hippie end of the spectrum, urbanite is a geological term. Urbanite refers to all those rocks that come out of old buildings, roads, and sidewalks. Generally, this means broken chunks of concrete, but it may also include tarmac, brick, paving stones, and other such debris. You may also see some downright classy urbanite, such as retro brickwork and cool tiles.
Urbanite can be used for foundations, low walls, garden paths, patios, filler, and various other DIY and green building projects. Generally urbanite isn't the best building material, but it is very cheap (if you are paying for it, you are doing something wrong), and it takes waste and makes something useful out of it, which is always good.
Harvesting urbanite is easy in theory, but hard work in practice. Look for buildings being torn down, local landfills, and empty fields full of piles of broken urbanite. I strongly recommend that you ask before you take, but generally people are happy to have someone haul the stuff away. (They may even drive it to your house and dump it for you; this saves them a dumping fee. Just be careful that you don't end up with more than you can use.) Landfills often have separate areas for urbanite, and while they may not encourage people digging through it, they will not consider you taking it as theft.