How does the away goal rule work?

Answer 1

The away goals rule is a method of breaking ties in association football and other sports when teams play each other twice, once at each team's home ground. By the away goals rule, the team that has scored more goals "away from home" will win if scores are otherwise tied. This is sometimes expressed by saying that away goals (or at least one of them) "count double" in the event of a tie. The away-goals rule is most often invoked in two-legged fixtures, where the initial result is determined by the aggregate score - i.e. the scores of both games are added together. In many competitions, the away goals rule is the first tie-breaker for such cases, with a penalty shootout as the second tie-breaker if each team has scored the same number of away goals. Rules vary as to whether the away goals rule applies at the end of normal time of the second leg, or after extra time.

Answer 2

In example in a champion league game, it Barcelona play A.c.Milan and score a goal. And in the return leg it ends0-0 . Then Barcelona will win on a away goal.