Despite its name, weak force is not the weakest of the four fundamental forces of nature. It’s much stronger than gravity, and it causes the kind of radioactivity that generates power within a nuclear reactor. It’s just weak because it can only operate within a very tight range.
Mainly, weak force operates something like a switch inside a quark. A quark is the smallest type of particle—small enough for every proton and neutron to contain three of them. In a proton, two of the quarks are switched up. Switch one of those quarks into a down quark, and—abracadabra— you’ve just made the proton a neutron. Even more impressively, if you’ve changed the number of protons or neutrons in the atom, you’ve created a completely different element. But wait, there’s more! While weak force is changing the particle, an electron or positron gets released, and that makes the process radioactive.