She is able to see big things that happen in the future. She has stated that she sees her kind best, Vampires, but if the human reason is big enough or she is close enough to them and has more of their tenor, she can see them as well aka Bella. Plus she can also focus on things she wants to see in the future. She can look ahead to see what happens to them in the future, or the weather like when they are going to play baseball. Most of the time she is focused at looking for the future of the Cullens, make sure that nothing unexpected or bad comes up. And then when Bella comes into their lives, she begins to look out for Bella. The reason Alice saw Bella when she jumped off of a cliff was because she was so attuned to watching out for Bella that even though she wasn't watching Bella's future anymore, it was a familiar wave that her mind automatically picked up on. Then in Breaking Dawn Alice probably wasn't watching their honeymoon, kind off gross, so she didn't pick up on the pregnancy right away, until the revelation came to Bella and Alice caught the big change. Now I hope that answer wasn't confusing. Alice can focus on what she chooses to see and recieves mostly visions on things that she selects to see the future off, in particular big changes. Bella jumping off a cliff, the vampires (Laurent, James, Victoria) Pregnancy. I love Alice!