Fusion releases energy when the nuclei of two forms of hydrogen (in our case, we use deuterium and tritium) are collided together at such high velocities that they stick together or fuse. Shortly after this, they break apart, forming a neutron and a helium nucleus. In this conversion a small amount of mass is converted to a large amount of energy, thanks to E=mc2, so the neutron and the helium nucleus produced by the reaction have a lot of energy.
In a fusion power plant, the neutrons will be used to heat water and drive a steam turbine. The helium atoms, which are positively charged do not escape the magnetic field so their energy is retained in the plasma, helping to keep it hot enough for fusion to continue.