Lieutenant-Colonel Virgil I. 'Gus' Grissom, USAF, Lieutenant-Colonel Edward H. White II, USAF, and Lieutenant-Commander Roger B. Chaffee, USN, were performing a 'plugs-out' test of the capsule, in which it was disconnected from external power and resources and functioned on its own internal systems. As the subsequent inquiry would show, there were serious flaws in the capsule's design, not least of which was, as Wikipedia points out:
Following a worldwide survey of artificial oxygen-rich environments, it was found that rarely if ever had a 100% oxygen environment been created and maintained at such a high pressure, in which a bar of aluminum can burn like wood.
It was never determined precisely what started the fire. Many defects in design and workmanship probably contributed. Its results were tragic. All three astronauts died within seconds.
Following the tragedy, numerous improvements and modifications were made to the Apollo capsule, subsequent examples of which went on to reach the Moon and enabled astronauts to land there.
Today, Launch Complex 34 at the Cape Canaveral Air Force Station in Florida is deserted, most of its structures demolished. Two plaques adorn one of the base pillars.
Let us remember their sacrifice with all honor.