what does [Dimension_Name].[Dimension_Name].[Dimension_Name].Members mean ?

I'm new to MDX, and many times while reading a book about MDX or some examples on the internet i've encountered something like:

[Dimension_Name].[Dimension_Name].[Dimension_Name].Members

for example :

[Employee].[Employee].[Employee].Members

in the following query

WITH MEMBER [Measures].[Employee Count] AS
	COUNT( NonEmpty( { [Employee].[Employee].[Employee].Members }, ( [Measures].[Project Hours] ) )	)
SELECT
	{
		[Measures].[Employee Count]
	} ON COLUMNS,
	NonEmpty(
		{ [Project].[Project].[Project].Members },
		( [Measures].[Project Hours] )
	) ON ROWS
FROM
	[Project Cube]

when ever i try such thing in my MDX queries i got the following error:

#Error Query (1, 61) The MEMBERS function expects a hierarchy expression for the  argument. A member expression was used.

what does something like [Dimension_Name].[Dimension_Name].[Dimension_Name].Members mean ?