Your output has 2 sections beginning with the word top and we want to print the second one.
After we met the second top i has the value 2.
i==2 is a condition here for awk, if it's true awk print the record per default so awk begins to print after we've reach the second top.
i==2 is similar to {if i == 2 {print $0}}

Have a read of an awk tutorial, you can find some links here:

Unix Tutorials/Programming Tutorials/Shell Scripting Tutorials

Hope this helps.