It is a function is SQL Server. It returns a value formatted with the specified format.
option of culture is available in SQL Server 2017.
Use the FORMAT function for locale-aware formatting of date/time and number values as strings.
DECLARE @d DATETIME = '11/10/2017';
SELECT FORMAT ( @d, 'd', 'en-US' ) AS 'US English Result'
,FORMAT ( @d, 'd', 'en-gb' ) AS 'Great Britain English Result'