creatine is the energy your muscles look to for first, it gives the muscles short bursts of energy , your body produces creatine naturally in the liver and kidneys , that is the non-scientific description
Also creatine works by increasing the water retetention in your muscles for a "bulkier" look so when u stop , your muscles shrink back to its original size.
as for being safe , studies show that it is safe
but do remember that it is a supplement not a miracle cure , Learning to train, eat and recover correctly are what needs to be focus on