Friends
Friends, unlike Decentralized Chat, allow you to establish persistent relationships with other BiglyBT users.
Each BiglyBT install has a unique public and private key pair that is used to both identify the client and also encrypt communication between that client and another - in the future this may be extended to support alternate identities for communicating annonymously, but until then remember that your public key is linked to your public identity on the internet.
When you and a friend are connected to the same download you can boost each other's download speeds. When this is occurring you will see an indication in the status bar:
As your identity is important to preserve (other friends will have this in their configuration) you should regularly backup your BiglyBT data (See Tools->Options->Backup & Restore). You can also separately export your public/private keys under Tools->Options->Security.
If you lose your keys, and you have no backup, then you will need to go through the annoying process of re-establishing relationships with all of your friends.