The Error Console will show internal GIMP error messages. An example of such an error is when GIMP fails to save or load an image.
If you encounter a bug in GIMP, it can be wise to open the error console and try to reproduce the bug. If GIMP then produces any error messages, you can email <bugs@gimp.org> the message along with a bug description to the GIMP developers at <gimp-developer@scam.xcf.berkeley.edu>.