Hi Joe,
Since 1996 all cars in the US are required to conform to the OBD-II specification, which mandates a standard connector provided that is located near the driver's seat. The check engine light is required to be illuminated when certain errors are detected by your engine computer, and there are a lot of them.
An OBD-II scanner is required to read the data (DTC's - Diagnostic Trouble Codes) out from the port; you can pick up bluetooth models that will connect to an app on your phone for under $10 USD, and the price can go up to several thousand dollars for professional grade equipment.
Fortunately, at least here in the US, most auto parts stores will scan the trouble codes and give you a copy of them for free. They'll bring their scanner out to your vehicle, hook it up and scan the codes for you.
So yes, it could indeed be something as simple as a loose gas cap, or it could be your emissions, or any one of several hundred other codes that can turn on the MIL, or Malfunction Indicator Light AKA the Check Engine light. It could be something minor that you can ignore, or it could be an indication of imminent catastrophic engine failure.
Scanning the codes is the way to find out why that light is on.