Lactic acid is a by-product of glycolysis. Lactic acid inhibits glycolysis and blocks the resynthesis of ATP through the combination of ADP and creatine. By delaying the accumulation of lactic acid, creatine allows the muscles to continue working with less pain and fatigue. Admittedly, this is a bare-bones simplified explanation. The bottom line is that more creatine in your muscles allows them to contract harder and longer, especially in activities involving short bursts of explosive energy, such as weightlifting, bodybuilding and sprinting.
Nothing works for everyone and creatine is apparently no exception.
The mean concentration is 125, which means that approximately half of people have less than that amount and the other half more. Strict vegetarians tend to be substantially lower than the mean. I'm a near-vegetarian. That's probably why creatine worked so well for me. Published studies show that athletes with concentrations below the mean respond positively to creatine supplementation, while athletes with elevated levels show little or no ergogenic effects.