Currency in Myanmar is called Kyats. These notes were printed long long ago and there is no exchanging old to new notes policy in place for so many years, so people are facing with shortages of small changes. Sometime you will get tissue packets, instant coffee mix, candy as a change.
As I heard from people who recently came back from Myanmar, the situation is not that bad right now (2012). This small change shortage is no longer applicable and you will see plenty of new notes in your trip.