[KoftaBob]

To put it simply, a huge amount of energy is used to produce so much heat and pressure, that the following occurs:

For every 1 deuterium nucleus involved, it fuses with 1 tritium nucleus => producing 1 helium nucleus, 1 free neutron, and 17.6 MeV of energy.

The energy released then causes more deuterium and tritium to fuse, producing more energy, and so on, in a chain reaction.

The goal, in order for this to be used to produce electricity for consumption, is for us to be able to kick start this process by introducing less than 17.6 MeV of energy per deuterium-tritium pair, so that after you account for the energy spent to fuse those atoms, there's a net energy left over for us to use.

Put even more simply, we're throwing heat/pressure at atoms, to cause them to fuse, and in the process convert some of that mass to energy.