the weak force
The weak force is the third of nature's four fundamental forces, and it is the one responsible for change. Where electromagnetism mostly pushes and pulls, and the strong force mostly binds, the weak force lets one kind of particle turn into another. It is the reason a radioactive nucleus can transform, the reason the Sun can burn its hydrogen into helium, and the only force that touches the elusive neutrinos.
What makes the weak force special is that it can change a particle's identity. Through the weak force a neutron can become a proton, a down quark can become an up quark, and a muon can become an electron. It does this by passing a heavy carrier particle — a W boson or a Z boson — between the particles involved. Because those carriers are so massive (roughly 85 to 95 times the mass of a proton), the weak force can only reach across an exceedingly tiny distance and acts feebly at the scales we usually meet, which is exactly why it is called 'weak'.
Despite its faintness, the weak force is indispensable. Without it the chain of reactions that powers the Sun could not even start, the chemical elements built inside stars and supernovae could not form, and radioactivity would not exist. In the modern theory it is not really a separate force at all: at high energies it merges with electromagnetism into a single electroweak force, one of the great unifications of twentieth-century physics.
The Sun shines because of the weak force. Its first and slowest step fuses two protons into a deuterium nucleus, which requires one proton to turn into a neutron — a weak-force transformation. That single sluggish step is so rare that it sets the Sun's pace and lets it burn steadily for billions of years.
The weak force throttles the Sun's fusion, making its long, steady lifetime possible.
'Weak' refers to how feebly it acts at everyday distances, not to any lack of importance; at very high energies its intrinsic strength is comparable to electromagnetism's.