Watchers are psychics who have Precognition, or the ability to see the future. However, since the future is fluid
Cassie shows Her and Nick's Death.
and always changing, Watchers aren't always 100% reliable (but usually still rank around 95-98%), due to the actions taken to garner a different result, and have to be vigilant to see all the changes to the future.
More skilled Watchers develop their skills to degree where they can see the necessary choices, actions, and happenings, in order to gain the desired future. Alcohol can enhance a watchers powers.