Deuterium is an isotope of hydrogen containing one proton and one neutron, and has a mass of 2. It is present in small quantities in seawater.
It is an important fuel for nuclear fusion, along with tritium. It can be formed in the sun by a process called beta decay in which two hydrogen nuclei are converted into a dueterium atom as one of the protons sheds its charge, becoming a neutron, plus a positron and a neutrino. This is usually the first stage in stellar fusion, which is then converted in to helium.
To clarify on this: almost all of the deuterium found outside stars was formed in the big bang, but stellar fusion produces deuterium by beta decay then destroys it by fusion, unlike other isotopes created in stars, it is not scattered by supernovae as it is immediatly used in fusion.