A Deuterium-Deuterium reaction produces Helium-3 and a neutron, or Tritium and a proton. Roughly even odds on which products you'll get. And a ton of energy. A Deuterium-Tritium reaction produces Helium-4, a neutron and a shitton more energy.

Basically, the DD reaction is used to create fuel for a more predictable and energetic reaction. From the article, seems they're also bombarding Lithium with neutrons from the DD reaction to create more Tritium. Problem with Tritium is that while it does occur naturally, it's extremely rare and volatile. Tritium has a half-life of about 12 years, so if we want to use it in a fusion reactor, we need a feasible way to produce more of it at scale.

It's all stepping stones to a greater goal though. DD and DT reactions produce neutrons, which will make your entire reactor highly radioactive thanks to neutron activation. D-³He and ³He-³He reactions produce ⁴He and protons, which are actually able to be controlled by magnetic fields and won't irradiate your whole shit. But then sourcing ³He becomes your new biggest problem.