I found this news article that discusses an article from the same researchers: https://bigthink.com/surprising-science/fifth-force-nature
Abridged from the article: "The mysterious X17 subatomic particle is indicative of a fifth force of nature. Four fundamental forces of nature: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. The discovery of a fifth force of nature could help explain the mystery of dark matter, which is proposed to make up around 85 percent of the universe's mass."
If you imagine for a moment that particles are integers and "total count" is a conserved property, then if I slammed a "2" and a "3" together and directly observed a "4" coming out, I could infer that there was also an "1" particle somewhere, since 2+3=4+1 (and wouldn't claim that there was a "2" and a "-1", because we already know we can observe "2"s.)
Real experiments use a variety of conserved quantities such as: mass/energy, linear momentum, angular-momentum/spin, charge, lepton number, etc.
This paper describes looking at spontaneous electron/positron pair emission. Electrons and positrons exactly cancel each out for some conserved quantities (charge, lepton number) where they have opposite signs but not for others (mass/energy), so its possible for things in general to lose some energy and generate an e/p pair.
This experiment observes the transition of 'high energy' Helium (He4') atoms to 'low energy' (He4) Helium where the excess energy created an e/p pair.
The reported observations show a high concentration of e/p pairs generated in such a way ("angular correlation") that implies that there was an intermediate particle created with a specific energy. The authors assert that this is evidence for the existence of the X17 boson, which they also assert might be a carrier for a 5th fundamental force (in the way that photons are the carrier for electromagnetic forces).
This is the second experiment they claim has found evidence of the same kind of hypothetical particle. The previous experiment used the transition from "high energy" Beryllium (Be8') to low energy (Be8) that also generates e/p pairs.
If true, this would be a big deal as it implies physics outside of "the standard model" of particle physics. That's what gets people talking about dark matter (which is also outside the standard model).
More likely (IMO), there's some experimental issue (i.e. they recorded bad data).