Those indicate the status of the thrust reverser on the left engine: arm, unlock, and deploy. There are three more lights mirrored on the right side of the glareshield for the reverser on the right engine.

The thrust reversers are deployed after touchdown to help the plane slow down, in conjunction with wing spoilers and wheel brakes, the latter of which become more effective as the aircraft slows, produces less lift, and has more weight on the wheels.