In data Warehouse when u change + symbol to - , u would c all the aggregation level in that + this is called rolling down.

So if u again click over that - it would again compressed back to +. This is call drill up.

your Aggregation levels woudl b like

Year>Month>Week>Day>Hour>Minutes>Seconds.

So by changing the symbol from - to + at month aggregation level u can drill up.