Weeder classes "weed" people out of majors that they do not belong in or are likely to do poorly in. ECS 30, 40 and 60, when taught by Sean Davis, are examples of weeder classes. Some people avoid "weeder profs" like the plague and manage to squeak by, but don't do so well later on.
ECS 222A is a fine example of a graduate weeder class. If you survive this class, you should have no problem with any other grad ECS class.
Weeder classes can also be found in the Biological Sciences. Basic weeder classes help people realize that they really don't want to go to Med School or do labwork.
Chem 2A, B, C are seen as weeder courses, but all of the upper division chemistry classes are much harder. All upper division organic chemistry classes can be seen as weeders; 118A, B, C for biosci people and 128A ,B, C for chem and chem engineering students.
Physics 105A, B and 110A, B, C are weeder courses for Physics majors/minors
Mathematics 108 used to be a major weeder class for math majors and others who wanted to minor in math. However, 108 is no longer required so the math department decided to take introduction to abstract mathematics and linear algebra and put it into one class, MAT 67, to make the new weeder class.
In humanities, Political Science 3 taught by Zeev Maoz could be seen as a weeder class as it requires a extensive reading and a lengthy term paper.
After going through these courses, the people taking the classes beyond them are usually seen as being pretty serious about what they are doing.