The technical term for the lights on the front of your Brother machine is Light Emitting Diodes (LED's).
This FAQ explains the meaning of those different LED indications.
When all the LEDs are off, the machine is in Power Off mode.
When the Ready LED is dimmed (Green), the printer is in the Sleep mode (Power Save mode) or the Deep Sleep mode (reducing more power consumption than Sleep mode). Receiving a print job will wake the machine and it will start printing.
The Toner (Yellow) LED is flashing.
The Ready (Green) LED is on.
The Drum and Paper LEDs are off.
Meaning of LED Indication:
Toner Low
If the Status Monitor shows Toner Low you can still print; however, the machine is telling you that the toner cartridge is near the end of its life.
Order a new toner cartridge now so a replacement toner cartridge will be available when the Status Monitor displays Replace Toner.
Out of Memory
The machine's memory is full. Reduce the print resolution.
Size Error DX (For automatic 2-sided printing models)
The paper size specified in the printer driver is not available for automatic 2-sided printing or the paper in the tray is not the correct size and is not available for automatic 2-sided printing. > Click here to see how to resolve the issue.
Size Mismatch
The paper in the tray is not the correct size. Load the correct size of paper in the tray and set the size of paper to the tray.
The Toner (Yellow), Drum (Yellow) and Paper (Yellow) LEDs are flashing.
The Ready (Green) LED is flashing.
Meaning of LED Indication:
Self-Diagnostic
The temperature of the fuser unit does not rise to a specified temperature within the specified time.
--OR--
The fuser unit is too hot.
Press and hold (Power On/Off) to turn off the machine, wait a few seconds, and then turn it on again. Leave the machine idle for 15 minutes with the power on.
Print Unable XX
The machine has a mechanical issue.
Press and hold (Power On/Off) to turn off the machine, wait a few minutes, and then turn it on again.