The category in which this one will be placed. 'Top' means that this category is not contained in any other category.
Normally you will see several category 'contexts' which you will see in bold type, notice that each context contains it's own category hierarchy. See below for more info on contexts. If you do not see several contexts then it may be because you do not have permission to access other contexts.
If there is only one category in a context, you will not be able to move the category as there must be at least one category in each context.
See also :