Monthly data conversion to Annual average?

Hello all...

I am trying to take a data set that has data for each month from 2011 to 2014. I was asked to convert the data to annual averages. Overall I need to be able to track the growth rate and create a trend line to 2018.

I am not certain that a straight average will be representative. What do you think?

Scouring around I found this formula:
=AVERAGE(INDEX(B:B,(ROW(A1)-1)*12+1):INDEX(B:B,ROW(A1)*12-1))

I don't really understand what this formula is doing. Would you translate?

Any input would be greatly appreciated!

Here is the data if it makes it any easier to explain.