Definition: Multivalued dependencies
occur when the presence of one or more rows
in a table implies the presence of one or more other rows in that same table.
For more detail, read What is a Database Dependency?
For example, imagine a car company that manufactures many models of car, but always makes both red and blue colors of each model. If you have a table that contains the model name, color and year of each car the company manufactures, there is a multivalued dependency in that table. If there is a row for a certain model name and year in blue, there must also be a similar row corresponding to the red version of that same car.