bitAssets are "shorted" into existence. what does "short" mean? someone who "shorts" is borrowing money and then selling it. why would anyone do that? if you believe the price of bread is going down tomorrow, you can borrow 10 breads today, and then sell them, ending up perhaps with 10 dollars. it turns out you did a clever move, because the very next day bread is worth half. since you promised to pay back 10 breads and not 10 dollars you can now pay back exactly what you borrowed, 10 breads, for only 5 dollars, leaving you with 5 dollars profit.
when you borrow something, the person you borrow from wants to make sure you don't run away with their money.. so they may ask you to leave something valuable or important that they can hold on to for security, for Bitshares it holds on to 3x the value of the bitAsset you are borrowing in BTS. so if you want to borrow 2 dollars, it holds 6 dollars worth of BTS as security until you pay the 2 dollars back.