It's between two isotopes of hydrogen, two kinds of hydrogen: deuterium, which is heavy hydrogen, which you can get from seawater, and tritium which is super-heavy hydrogen.
是氢原子的两个同位素之间的反应，两种氢原子， 这两个同位素就是氘原子， 一种可以从海水中获得的重氢元素， 以及氚原子，一种超重氢元素。