ECID stands for Exclusive Chip ID or Electronic Chip ID and it's a number unique to each device. You may be asked for your ECID when saving SHSH blobs.
Connect your iPhone to your computer via a USB cable and launch iTunes.
Click the iPhone icon at the top left of the iTunes window to view your device.
Make sure you are on the Summary screen and then click the serial number of your device twice to reveal its ECID.
You can now use Command+C or Control+C to easily copy the ECID to the clipboard.