REF_ROWS("Order Details", "Order ID") gives a list of rows from the Order Details table whose Order ID column value matches this row's key column value, where "this row" is the row from which the REF_ROWS() expression is evaluated.
REF_ROWS("Order Details", "Order ID") is functionally equivalent to each of the following:
FILTER("Order Details", ([_THISROW] = [Order ID]))
SELECT(Order Details[Order Details ID], ([_THISROW] = [Order ID])).
REF_ROWS( "from-dataset" , "ref-column" )
from-dataset (table or slice name): The name of the table or slice (the "data set") in which related rows are to be found.
ref-column (name of column in from-dataset): The name of a column in the data set named by from-dataset that contains Ref-type values.
Note that both arguments must be raw text values (quoted or unquoted) that identify an existing table or slice (from-dataset) and column (ref-column); they may not be expressions.
The answer to the question: which rows in from-dataset refer to this row from ref-column?
A list of key column values from rows in the specified table or slice (from-dataset) in which the specified Ref-type column (ref-column) refers to this row (the row from which the REF_ROWS() expression is evaluated).
If a column's App formula consists exclusively of a simple REF_ROWS() expression, a row added from the resulting list's inline view will automatically be populated with a reference back to the row of the REF_ROWS() expression, creating a child-parent relationship. The parent reference will be placed in the column named by the ref-column argument of the REF_ROWS() expression.