When doing photography in a dark environment with dslr camera in auto mode, the pop up flash gets activated automatically. This is because the brain of camera thinks it is too dark, and requests help from pop up flash to throw in some light. Most of the entry and mid level dslr photography cameras have an on board flash system from the manufacturer itself. Example: Canon Rebel series, 20D, 30D, 40D, etc.
FEC(Flash exposure compensation) is the ability to adjust output power level of a dslr flash unit. The need for flash exposure compensation is due to the fact that the internal flash can cast really harsh shadows or can even lead to an over exposed subject. The adjustment of power level allows a better control over the lightning which leads to properly exposed subject.
For most of the canon dslr cameras, flash exposure compensation value can only be changed in manual, program, aperture priority, and shutter priority modes. The images below were photographed in a totally dark room with the external Yongnuo flash unit as the subject.
Image 1: FEC -2
Image 2: FEC Zero
Image 3: FEC +2
The impact of changing flash exposure compensation is clearly visible in above 3 images. Image 3 has the highest value of flash exposure compensation, and that's why it is the brightest.
Use: This flash exposure compensation technique can be used to assist in filling shadows for outdoor portrait photography. Another use can be for indoor events. Increase the exposure compensation and use a bounce card or cover to reflect it from the ceiling. This would light up the whole area with a very pleasing light. Bouncing flash prevents harsh reflections and over exposed edges while doing indoor photography.
Pro Tip: Camera retains the flash exposure compensation values even after switch off, restart. Let's say you finished a photography session with fec+2 and switched off the dslr, keep in mind that this value would be retained next time whenever a photograph is taken.