BLF or busy lamp fields show the status of other users phones (busy, ringing, available) connected to your phone system. Required is that you have this user on one of your BLFs and that your phone has BLFs in the first place. Modern IP phones use multi colour LEDs to indicate the status of another phone. BLFs can also be used to call other users by pushing the BLF button.
Some phone systems also allow to do pickups of the phones of other users. This way you can pickup calls from your users when they are not available. Usually users need to be members of the same pickup group, in order to pick up each others phones.
Pickups can be combined with BLF. When the phone of another users is ringing, this is indicated by a blinking BLF on your phone. If you and the other user are in the same pickup group, you can pickup the phone by pressing the BLF button on your phone. You can watch an example, how to pickup a call in this video.
AskoziaPBX fully supports BLF and Pickups with a variety of different phones. More information about the configuration can be found here.