SET Identity_insert - allow to be inserted explicit values into the identity column of a table.
The IDENTITY_INSERT statement must be set ON to insert explicit value for identity column.

SET Identity_insert Syntax:

SET IDENTITY_INSERT [ database_name . [ schema_name ] . ] table { ON | OFF } ;

SET Identity_insert Example:

USE model;
GO
CREATE TABLE Department(
ID INT IDENTITY NOT NULL PRIMARY KEY, Name VARCHAR(250) NOT NULL);
GO
INSERT INTO Department(Name)
VALUES ('Anthropology'), ('Biology'), ('Chemistry'), ('Computer Science'), ('Economics');
GO
DELETE FROM Department WHERE name='Biology';
GO
SELECT * FROM Departments;
GO

USE model;
GO
INSERT INTO Departments (ID, Name) VALUES (2, 'Biology');
GO

USE model;
GO
SET IDENTITY_INSERT Departments ON;
GO

USE model;
GO
INSERT INTO Departments (ID, Name) VALUES (2, 'Biology');
GO