Dinner — also called supper — is the third meal of the day. It's the main meal, the one families often try to have together at the end of the day.
If you skipped breakfast and lunch, then you probably can't wait for dinner: you must be starving! Dinner has always been the main meal, but it used to be served in the middle of the day and supper was last. Now that dinner is in the evening, too, supper and dinner are almost the same thing in the United States — you might go to a dinner in honor of someone, but not a supper, which wouldn't be as fancy.