In a very oversimplified nutshell (leaving out theory etc), it means to contract the abdomen on the inhale and expand (or release) the abdomen on exhale. The "normal" way is the to expand on the inhale and contract on the exhale.
Here is a link given in another thread on this same subject:
http://tinyurl.com/pdqe6
with a lot more explanation.
For me right now, I am feel very liberated to just focus on the movements, the flow and natural breathing. Later, once I master the movements, I will learn reverse breathing a little at a time. I have found that in a lot of things, and most especially things like Qigong and Tai Chi Chuan, it is usually better to take things slowly and gradually.
