Auto-embed links allow the user to have the ability to automatically embed any Youtube videos, Twitter tweets, Imgur images, Vine videos, and Instagram photos directly into topics and replies. This is powered by the oEMBED feature inside of WordPress. For a full list of what oEMBED supports in bbPress and WordPress go here.
To enable the ability to use oEMBED on bbPress topics and replies:
Settings > Forums in the WordPress backend
Forum Features section make sure “Embed media (YouTube, Twitter, Flickr, etc…) directly into topics and replies” is enabled