It can mean any number of things.
Assuming that you are using the OS X 10.6.x operating system on your Mac:
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Extreme, then click Manual Setup
Click directly on the word Status (2nd line on the summary page)
A window will open to explain why the amber light is flashing, along with suggested actions(s)
If you are using Lion or Mountain Lion:
Open AirPort Utility as above
Click directly on the AirPort icon
Look for Status
Click on the amber dot
It means you are connected, but their is an issue requiring your attention. Bob is corrent follow his lead.
If you cannot relove the issue, you should reset. Here is the article to reset the Airport.