What does the red light indicate on a Raspberry Pi?

What does the green light indicate on a Raspberry Pi?

The original Raspberry Pi has extra green lights

If you're lucky enough to own an original Raspberry Pi, there are some additional LED lights to be aware of. The extra LEDs include two green lights and a yellow light, all of which are related to the Raspberry Pi's network connectivity status.

The first of the green lights is labeled "FDX", which stands for Full-Duplex connection. This type of ethernet connection is used to allow the simultaneous sending and receiving of data between two devices without collisions. If the FDX label is lit, it indicates your Raspberry Pi has an active network connection and is capable of sending and receiving data.

The second green light has the label "LNK" and is lit during network activity. Whenever there's any type of traffic on the network, such as file transfers, the LNK light should flash green. Not seeing the LNK light flash green is a good indication that there's a problem with the network configuration or that a faulty cable is being used. Lastly, linked to the functionality of the green lights, you'll also find a yellow light labeled "10M" or "100". This LED is lit when your Raspberry Pi has established a connection of 100 Mbps. The extra yellow and green lights have been cut from later models, so you'll only find them on the classic models manufactured before the Raspberry Pi 2.