what is the use of padindex function

Returns the starting position of the first occurrence of a pattern in a specified expression, or zeros if the pattern is not found, on all valid text and character data types. syntax : PATINDEX ( '%pattern%' , expression )

I'm confused.

Do you mean the PATINDEX function, which, as @viikiseth has indicated, is for string pattern matching - see also http://msdn.microsoft.com/en-us/library/ms188395.aspx

Or did you mean PAD INDEX, which you can use to tune index performance - see http://msdn.microsoft.com/en-us/library/ms186872.aspx

I'm guessing, by the way you described it as a function, that you meant the former...