Lock Boxes, are loot drops with random chance items inside that could either be that top level purple piece you are looking for, or something like say, a common medpac.
THe box drops off of any mobs, doesn't even have to be elite or anything, HOWEVER you must buy the key to the box off the real money store.
You don't have to though. In sto people can get keys on the exchange too, spending no real money.
Also, the items from lockboxes are sold on the exchange, so again, there's no need to spend real money. I keep seeing so many people say how easy it is to earn credits in this game, so buying lockbox items from the GTN should be easy enough.