The Super Soldier Serum is a combination of special chemicals created by Professor Abraham Erskine and his team of scientists and chemists that created Captain America during World War II in the Operation Rebirth.
The history of this special serum is well-known because it was created by Professor Erskine in the secret U.S. military program called Operation Rebirth that creates an army of Super Soldiers out from any ordinary men in order to gain Allied advantage over Hydra during the war.
The Super Soldier Serum had worked on stick-thin Steven Rogers that gained him a battle-ready Super Soldier battle-ready muscular build.
The result was a success but it can only be used once. However, the Super Soldier serum and the knowledge of creating it from Erskine who died vanished when the war ended with Allied victory after Cap's and Bucky's disappearance.
Over the years, S.H.I.E.L.D., HYDRA, A.I.M. and other organizations, nations, companies, etc. had tried to recreate the Super Soldier Serum in different versions to create their own Super Soldiers to command but it result to creating different super-powered beings (heroes and villains) instead.
Since nobody else can recreate the perfect Super Soldier Serum, it means that Captain America is the only true Super Soldier left in the world.