A thermonuclear explosion is a powerful release of energy resulting from nuclear fusion, typically involving isotopes of hydrogen, such as deuterium and tritium, under extreme temperature and pressure conditions. This process is the principle behind hydrogen bombs, where an initial fission explosion triggers the fusion reaction, leading to vastly greater destructive power compared to fission bombs alone.
Hiroshima is a city in Japan where, a long time ago, a big bomb called an atomic bomb was dropped during a war, causing lots of damage and sadness. This event made people realize how powerful and dangerous these bombs can be, and it helped the world understand the importance of peace and not using such weapons again.