Getting the exposure right for flash photography used to involve time-consuming calculations based on guide numbers, flash-to-subject distance and apertures. Thankfully, evaluative through-the-lens (E-TTL and E-TTL II) flash metering has changed all this, and Speedlite flash photography is now as easy as pressing a button. But even so, you'll sometimes get better results by giving the flash metering a little help.
Canon EOS cameras' evaluative metering system, which is used to take ambient exposure readings, is shared with the E-TTL flash system. A low-power pre-flash fires in advance of the exposure, and the light from this flash is metered. The duration of the main flash is then controlled to give correct exposure with the selected aperture.
When you are shooting with daylight, exposure adjustment is sometimes needed with very light or very dark subjects. This is because the camera is calibrated to suit an "average" scene. You can either take a reading from an "average" area of the scene and use the exposure lock function to hold this for the main scene, or use the camera's exposure compensation function.
Since E-TTL and E-TTL II flash use the same metering system, the flash meter readings can also be confused by light or dark subjects. In this case, the adjustment is made using the flash exposure lock (FEL) or flash exposure compensation (FEC) functions.