of a group from the comics called "The Scavengers" from the D.C. area who were pretty much a few people who would attack/steal things from different communities.

In the show, they were teased all throughout the second half of season 5. They were the group responsible for destroying Noah's community (There was the "Wolves not Far" quote spraypainted throughout the area in the background). They had a bunch of random walkers around Alexndria with "Ws" carved into their foreheads as well that let everyone know the "wolves were not far".

As far as their origin, it was pretty much explained in Morgan's first encounter with the leader in last season's finale. They're just a bunch of ruthless scavengers who will kill anyone in their path and take everything they have.

I wouldn't say that they weren't a threat. When they attacked Alexandria, it came outof nowhere and caught the community off guard. They were a prime example of what Rick was preaching to the community of why it's important to always be on your guard, what kind of threats live beyond the walls, how they need to toughen up, etc. Not to mention they were responsible for the other half of the herd getting to Alexandria and eventually inside the walls.