This is the actual live optical power this transceiver is putting onto the fiber. The admin compares it against the transceiver's rated output spec and the low Tx-power alarm threshold to judge whether the laser itself, rather than the fiber path, is the source of a weak signal reported at the far end. For example, a reading of 900 microwatts on an optic rated for roughly 1000 microwatts nominal output is essentially healthy, whereas a drop to 200 microwatts would point to a failing laser inside this specific transceiver.