Fusion requires a lot of heat, because the point is to be atoms flying around really, really fast, so when they smash into each other they stick together and make a bigger atom, instead of bouncing apart. Heat is really a measure of atomic activity (meaning, lots of heat makes atoms move very fast). So the idea behind the fusion reactor is that if we get a certain area hot enough, all we have to do is keep adding fuel and it will sustain the reaction. (think of it as a normal wood fire, only instead of chemical energy being freed from the wood, its atomic energy being released from sea water. Once you get a wood fire going, all you have to do is add wood, same idea with the sea water.) So that's how you get energy from fusion.
The issue is that the temperatures required will melt anything that is in contact with it. The solution is to use magnetic fields to hold all the fuel away from the walls of the container. A toroid (picture a donut, with a magnetic field flowing inside of it) is the best way to do it, because its fairly easy to make one (basically you take a solenoid and stick both ends together to make a circle) and its a more stable field than trying to get a spherical container.
Hope this helps!