RAISERROR

Creates an error message and starts the session’s error processing. RAISERROR can create a message dynamically or reference a user-defined message saved in the sys.messages catalog view. The message is returned to the caller program as a server error message or to a TRY…CATCH construct’s associated CATCH block.

In other, you can raise a custom error message with this keyword.

The example is shown below. Here I checking if the number is less than zero then raise an error.

BEGIN TRY

    DECLARE @NUM AS INT
    SET @NUM = 2 - 5
    
    IF @NUM < 0
    PRINT(@NUM)
    RAISERROR('Number Cannot be Negative',1,11)
    
END TRY
BEGIN CATCH
  
END CATCH