Why are multivalued dependencies named that way?
A multivalued dependency A->->B means that each value of A determines a set of values of B (and not a single value of B as in functional dependencies).

For instance, suppose that programmers have an attribute programmer-id and an attribute known-language, and each programmer can know several languages, you have the multivalued dependency:

programmer-id ->-> know-language

So, supposing that the programmer with program-id 7 knows SQL and Ruby, this means that in a table in which you have both attributes programmer-id and know-language, every time there is program-id 7 there must be two different rows, one with language 'SQL' and one with language Ruby, and all the other attributes equal:

programmer-id  know-language other-attribute1 other-attribute2 ...
    7              SQL           XXX               YYY
    7             Ruby           XXX               YYY
    8             Java           AAA               BBB
 ...