Thermonuclear fusion is a way to achieve nuclear fusion by using extremely high temperatures. So basically we can say nuclear fusion at high temperature will be thermo-nuclear reaction. Temperature is a measure of the average kinetic energy of particles, so by heating the material it will gain energy. After reaching sufficient temperature, given by the Lawson criterion, the energy of accidental collisions within the plasma is high enough to overcome the Coulomb barrier and the particles may fuse together.