What do the LED lights mean on an Ethernet port?

Most Ethernet ports will have two LED's, one on the left side and one on the right side. 

One will indicate connection state (Solid) and the other connection activity (Flashing).

Different devices or hardware manufactures may have these on either side

Connection state

This indicates the speed the device is running at. 

Amber = 1000Mbps.

Green = 100Mbps

No LED = 10Mbps

You'll notice in this last image that the LED on the other side is on, which brings us to below

Connection activity

This is used to indicate link activity.

This will simply flash whenever there is network activity, and will only be the one colour.