DIAGNOSIS That's the water shortage indicator light. It means the unit either doesn't have enough water in the reservoir or the sensor can't detect the water level properly.
FIX First, power the unit off. Pull out the water reservoir and check the water level. It needs to be above the MIN line, but don't fill it past the MAX line. Make sure you're pouring water directly into the reservoir, not the ice basket area. Slide the reservoir back in firmly until it clicks or seats completely. Power it back on. The light should go solid or turn off, and it'll usually start a cycle within a minute or so.
IF THAT DIDN'T WORK If the reservoir is full but the light persists, the float sensor inside the reservoir might be stuck. With the unit off and unplugged, carefully remove the reservoir again. Gently shake it and listen for a little plastic piece moving freely. You can also try rinsing the reservoir out with clean water to dislodge any minor debris.
HEADS UP Always use the unit on a level surface. If it's tilted, the water can pool away from the sensor, tricking it into thinking it's empty even when it's not.