Dax Question what does the . (dot) mean in this case?

[CharlesMS]

Hello everyone, 

Can anyone elaborate what does the . (dot) mean in this case :

VAR FirstD = LOOKUPVALUE(DATETABLE[DayN_Sequential],DATETABLE[Date].[Date],DATE(2022,7,1))

And can anyone tell me why I get an error saying Parameter is not the correct type?

I think its because the column of that table is not a hierarchy? 

Is there a way to convert the column to hierarchy? and how does hierarchy differ than the normal column ?

[CharlesMS · Accepted solution]

So apparently the .[Date] changes the format of the column you are selecting