Short answers, pulled from the story.
Command Pilot Gus Grissom, Senior Pilot Ed White, and Pilot Roger B. Chaffee died on the 27th of January 1967, during a launch rehearsal test at Launch Complex 34 on Cape Kennedy Air Force Station. A cabin fire broke out in the pure-oxygen pressurized spacecraft; all three astronauts died of cardiac arrest caused by high concentrations of carbon monoxide.
The review board identified an electrical ignition source most likely near a silver-plated copper wire stripped of its Teflon insulation near the Environmental Control Unit, combined with a pure-oxygen cabin atmosphere at 16.7 psi and extensive combustible materials including approximately 34 square feet of Velcro. The plug-door hatch design prevented escape because it could not be opened against the high internal pressure.
The inner hatch used a plug-door design that sealed more tightly as internal pressure increased. Procedure required Grissom to open a vent valve before White could remove the hatch, but that valve was located behind the initial wall of flames. The fire also caused cabin pressure to surge from 16.7 psi to 29 psi almost instantly, far beyond the system's ability to vent.
The Phillips Report was a document prepared in late 1965 by Apollo Program Director Maj Gen Samuel C. Phillips, documenting serious cost overruns, schedule delays, and quality failures by North American Aviation on the Apollo command and service module and Saturn V second stage. NASA administrator James E. Webb was unaware of the document; Senator Walter Mondale publicly revealed its existence during the 27th of February 1967, congressional hearings, causing significant controversy.
NASA redesigned the Block II command module with a new outward-opening hatch that could be cleared in under five seconds, replaced nylon suit material with flame-resistant Beta cloth woven from Teflon-coated fiberglass, changed the cabin launch atmosphere to 60% oxygen and 40% nitrogen at sea-level pressure, and replaced flammable cabin materials with self-extinguishing versions. Crewed flights were suspended for twenty months while these changes were made.
Apollo 7, flown in October 1968, was the first successful crewed Apollo mission. Its crew -- Wally Schirra, Donn Eisele, and Walter Cunningham -- had served as the backup crew for Apollo 1. The mission flew on AS-205 in a redesigned Block II command and service module.