The SI board in the cluster sends pulses through the cluster into your eye and it bounces back into the cluster and the light will come on if the pulses find that you have baggy eyes or when your eyes are droopy for whatever reason. Another innovative feature from the 80s!
Im joking. Its the Brake lining indicator. It comes on when your pads are low or when the sensor circuit is closed for whatever reason.
'88 E30 325i sold
'88 E30 325i 2.7 stroker sold
'95 E36 318i sold
'94 E36 325ic Cabriolet sold
'87 Mazda RX-7 Turbo II sold
'96 E36 328i Cabriolet - sold
'92 Mazda Miata Black Nardi Edition sold
'98 E36 M3 Cabriolet -
"Float like a Cadillac, Sting like a BEAMER" ... ... Lightning McQueen
It's not really anything to worry about bmw seems to place the sensor in the pads extremely deep so these lights usually go off prematurely. If that light just came on you are likely only at 65% pad wear.