Dell optiplex 3019

Hi everyone.

What is the sign of the yellow light on dell optiplex 3010 motherboard(what is the signal of it) ?

.The desktop power button doesn't show any light and computer doesn't turn on when I press the power button.

2. Inside the mother there is yellow light.

3. When I press the power supply button at back, the power supply respond well with the solid green light and all the fans are spinning very well.