Hi Jitendra,
First of all there must be SideNote defined range in the workbook, having atleast 2 or more columns.
So, basically this function is triggering a VLOOKUP function only if value in the cell O22 is greater than 0, other wise the formula will return a null string.
Now in VLOOKUP, this part IF(ISERR(AVERAGE(H16:H22)),0,ROUND((N22-N15)/N15,3)) is giving the lookup value, so if there is an error in calculating average of the range H16:H22, the lookup value will be 0, otherwise it is equal to change in N22 from N15 (i.e. percentage change) rounded to 3 figures.
Now this value is looked up in the range SideNote and 2nd column is return for an approximate match as the fourth argument is 1.
Regards,