MDX Query on SQL 2008 SSRS

For the MDX query below,

"FROM ( SELECT ( STRTOSET(@DIMDATEYearGroup, CONSTRAINED) ) ON COLUMNS"

Is it similar to the where query in TSQL, what the use of STROSET & CONSTRAINED ? The condition defined doesn't need to be a selection list in the query,  correct ?

Inaddition, what does the ON COLUMN mean ?

SELECT
NON EMPTY { [Measures].[Sales Last Year YTD], [Measures].[Sales YTD],[Measures].[Budget YTD] } ON COLUMNS,
NON EMPTY { ([DIM DATE].[Year Group].[Month].ALLMEMBERS ) } DIMENSION PROPERTIES MEMBER_CAPTION, MEMBER_UNIQUE_NAME ON ROWS
FROM ( SELECT ( STRTOSET(@DIMDATEYearGroup, CONSTRAINED) ) ON COLUMNS FROM [MyCube])
CELL PROPERTIES VALUE, BACK_COLOR, FORE_COLOR, FORMATTED_VALUE, FORMAT_STRING, FONT_NAME, FONT_SIZE, FONT_FLAGS