How the function is used trickily to get the desired result

ACTUAL DATA 

ALTER FUNCTION [dbo].[fn_Split_month]

(

       @Data varchar(max)

) 

RETURNS varchar(30)

AS 

BEGIN

       Declare @Month varchar(15),@Year varchar(4), @RtnValue varchar(30),@index int

       if(len(@Data)>0 and @Data<>'')

       begin

              SET @index = Charindex(':',@Data)

              SELECT @Month = month_text from tbl_month_master where month = ltrim(rtrim(Substring(@Data,1,Charindex(':',@Data)-1)))

              SET @Year = LTRIM(RTRIM(SUBSTRING(@Data,@index+1,LEN(@Data))))

                    

              set @RtnValue =  @Month + ', ' + @Year

       End

       Else

       begin

              set @RtnValue=''

       End

       Return @RtnValue

END

SELECT [dbo].[fn_Split_month] ('12:2009')