Busy Lamp Field, also called BLF, presence, busy light field, is a feature offered by some business-grade IP phones which indicates visually (and audibly on supported handsets) if other extensions on the phone system are busy, available or receiving a phone call.
BLF also allows calls to be transferred much easier, allow remote pick-up and can be simply used as speed dial.
Note that not all handsets offer BLF.
To set up BLF, please see the relevant guide for your handset: