This image depicts how a "blow hole" can develop in a space shuttle’s Solid Rocket Motor if the O-Rings fail and cause a joint to become misaligned.
In a normal configuration of a properly aligned joint, the blow hole, depicted in this image, is the place where sealing paste is applied.
If a blow hole exists, the hot gasses which are generated during ignition and liftoff have a place to escape from inside the Solid Rocket Motor and Solid Rocket Booster.
If hot gases escape, and damage the shuttle’s external fuel tank (which contains liquid oxygen and liquid hydrogen), a catastrophic explosion could occur.
Image from "The Challenger Accident: Mechanical Causes of the Challenger Accident," by the University of Texas. Online, courtesy NASA.
The linked illustration - by Kapitel - depicts a simplified cross-section of the field joints which existed between the assembled segments of the Solid Rocket Boosters which NASA used in the Challenger's STS 51-L mission. The following legend describes the drawing’s details:
A - steel wall thickness 12.7 mm
B - base O-ring gasket
C - backup O-ring gasket
D - Strengthening-Cover band
E - insulation
F - insulation
G - carpeting
H - sealing paste
I - fixed propellant
License for Kapitel’s illustration: CC BY-SA 3.0