Street Dates are fairly easy to understand, the problem is a lot of people don't know they exist or why they exist.
A Street Date is a specific date attached to an item of merchandise that the manufacturer does not want the item sold before.
Take Skylanders Trap Team for example. It had a Street Date of October 5, 2014 (here in the US). So while retailers had the game in their possession well before October 5th they were not supposed to sell it before that date.
Selling products before their Street Date can result in major fines, future restrictions (for the store or chain as a whole), or even lawsuits. It is in everyone's best interest to adhere to street dates.
It takes a long time for hundreds of thousands or even millions of copies of a game to be shipped out to all retailers. So in order to coordinate a scheduled release, the product must start shipping weeks in advance of the release date.
As the result of this process some stores will receive their product weeks before the release while other stores will only get it days before the release.
I can only imagine the chaos that would occur if stores were allowed to release products as soon as they were received.
Just about every new product will have a Street Date even if it's not a highly touted product. Anyone who's worked in retail, especially in the back room or warehouse should know what I'm talking about. (I spent about 4 years in the back room of major retailers.)
Most commonly you will here about Street Dates with music, movies, and video games, but these are not the only areas, they're just the most talked about.
Street Dates are your friend. They allow you to know when products are coming out so that you can plan ahead and not be left without.