Security Filter may not be the best approach for filtering the Orders as you describe. My thought is you might have other Orders that need to be reviewed or edited but are scheduled for delivery in the future.
Instead,a Data Slice named “Todays Deliveries” (or some such name) might be more appropriate. Then you can create a view using that slice which will only show Orders scheduled for delivery today - but as another view in the app as opposed to the app showing ONLY those orders to be delivered today.
Whichever approach you use, the expression would be the same. I’ll assume you have a column in the Order table named as “Delivery” that references the Deliveries table.
[Delivery].[Delivery Date] = TODAY()
Yep! That should be all you need unless of course you truly have other criteria to consider like cancelled orders that were scheduled for delivery today.
Now, to explain dereference. The period in the expression is the dereference operator. It can only be used on columns that are defined as Ref or have a sub-type of Ref (like in Enum or EnumList columns). With the dereference operator you can specify any column in the referenced table in order to access that column value. And you can chain dereferences such that you have [Ref Column].[Another Ref Column].[Still Another Ref Column].[Column for Value].
For more info on dereferences, check out the article below: