Hey MTR, let me try to explain it to you.
The Champions League games are played in 2 legs (1 home, 1 away) The thing you have to remember about this system, is that if the aggregate score (i.e. the total score in BOTH games combined) is equal, the tiebreaker rests on the team that scored more goals AWAY from home.
In the Chelsea/Liverpool matchup, the first game ended up tied 1-1, but it was played in Liverpool, and as such Chelsea has the advantage here because they scored a goal AWAY from home. That means a 0-0 game will put Chelsea through. Liverpool HAS to score at Chelsea if they want to advance (i.e. win the game by having a higher aggregate score line.) Of course, if the game in Chelsea ends 1-1, then it goes to extra time and penalties (as both the aggregate would be 2-2, and both teams would have scored 1 goal on the road)
That's still pretty steep of a price though. Anything can happen, but there is a distinct advantage given to the home team. Also to bear in mind here is that Chelsea remains unbeaten at home for 81 straight games now.
Hope this helps.