Fusion relies on the nuclei of atoms colliding. The first requirement therefore is to remove the atoms’ outer layers, their electrons, to expose the nuclei. This is done with heat and electric fields – the atoms get hot, and shed their electrons, creating a plasma soup of nuclei, which are positively charged, and electrons, which are negatively charged.
High-school physics teaches us that equally charged particles, instead of colliding, would repel each other. To overcome this repulsion, the nuclei need to collide at high speed, and this is achieved by heating the plasma, which makes all the nuclei whiz around faster. As the plasma gets hotter nuclei start to collide at high speeds, and a small fraction of them stick together, releasing a large amount of energy.