Why it matters.
Use it when the meter gives a result that does not match your intention.
A closer look.
Positive compensation usually brightens the result; negative compensation darkens it. The setting changed depends on the exposure mode and whether Auto ISO is active.
Picture this
Photograph a white object on a bright background. Try a small positive adjustment and inspect the highlights.
Keep in mind.
In fully manual exposure with fixed ISO, compensation may only change the meter indication. Check your camera manual.