Someone, or a group of someones is hiring Shadowrunners for items of interest from the Fifth World. "Items of interest being" closer to "weird collectables", but the job pays.
The Collectors are a group or people or an individual hiring Shadowrunners to acquire items that a buyer or a number of buyers are interested in. They are often the middle man in these transactions, making money off auctioneering or middle ground acquisitions.
Making money and hiring the runners for mutual benefit.
These guys are the folks who the runners are hired to get the items from, by hook or crook.
To not get stolen from.