HMB (short for β-Hydroxy β-Methylbutyrate) is a metabolite of the amino acid leucine that, along with KIC (α keto-isocaproate) and isovaleryl-CoA, mediate the effects of leucine. Approximately 5% of dietary leucine is oxidized into HMB, and HMB appears to be the main metabolite of leucine that more effectively prevents the breakdown of muscle protein.
When compared to leucine, HMB appears to be significantly more potent on a gram per gram basis at attenuating the rate of muscle protein breakdown but is less effective than leucine at inducing muscle protein synthesis. Due to this, HMB is marketed as an anti-catabolic agent (purposed to reduce the rate of muscle breakdown) rather than an anabolic agent (purposed to increase muscle mass).
Human trials don't normally tend to be structured to properly assess the effects of HMB, as most of the studies are a standard diet paired with an exercise regimen investigating the role of HMB in promoting muscle protein synthesis (of which it is similar to leucine in the sense that there are positive results, but quite unreliably so); the limited evidence that assesses HMB during periods of muscle loss are either underpowered or not in athletes.
HMB, currently, appears to be a pretty interesting supplement for the purpose of reducing muscle wasting during periods where muscle atrophy is accelerated (cachexia, AIDS, bedrest) and should theoretically work in athletes on a calorie restricted diet but is not fully established for this role yet (which is a notable issue, since glutamine has a large dichotomy between clinical and healthy populations).