Nuclear fusion is the process in which two lighter nuclei are merged to formed a heavier nucleus. This mass of the newly formed heavier nucleus is not equal to the the mass of the two lighter nuclei; it is lighter. The difference in mass is released as energy, according to Einstein’s equation: E=m * c^2, that is, Energy (E), equals mass (m) times the speed of light (c) squared! Simple as that.
It is quite difficult to achieve in earth because it required the two colliding nuclei to have very very high energy which means they need to have very high temperature. If we achieve to control nuclear fusion, we would however would have achieved clean energy because there are not much bad by-products, and this is a holy grail in modern physics!
A great example of nuclear fusion is our sun! The temperature at the center of the sun is so hot (over 10 million degrees) that nuclear fusion takes place – that’s why the sun emits that much energy and warms up our planet nicely(probably not in the UK!). In fact, the sun loses one million kilograms in mass every second because of nuclear fusion. Don’t worry though, it has plenty left and it will be there for a few billion years more at least!