What is Nuclear Fusion?
Nuclear Fusion is the process of joining two lighter nuclei together to make a heavier one.
The mass decreases in the process and is transferred into energy.
Nuclear fusion is the process that stars use to generate energy. In the core there are high pressures and high temperatures which are required to fuse nuclei. This is because the nuclei repel due to their positive charge. But when the nuclei are close enough, the strong nuclear force kicks in and overcomes the Coulomb repulsion.