Index array & sort algorithm???

Hello Everyone.
Just two questionns i wanted to know.

1) can someone explain what is meant by an index array and why its use can improve the efficiency of sorting?

2) Can someone give me a more efficient verson of the sort algorithm below with
reasons why its more efficient??

for (int p = 0; p<n-1; p++)
for (int counter = 0;counter < n-1; counter++)
if (a[counter]> a[counter+1])
swap(a, counter, counter+1);

All help apreciated.