What does it mean when a dataset is 'under embargo'?

When publishing a dataset, a user may choose to defer the date at which the data becomes available (for example, so that it is available at the same time as an associated article). This means that the description and files of that dataset are not publicly available until the embargo date is reached. Meanwhile, other information about the dataset, such as the contributors, title, citation and associated articles, becomes available immediately prior to the embargo.