Inertial navigation and gimbal lock
Knowing where you point and how you move from spinning gyroscopes and accelerometers alone, with no outside signal; three nested gimbals can jam.
Inertial navigation works out a vehicle's direction and position from the inside, using no stars, radio or GPS. Gyroscopes resist being turned, so a platform held steady by them keeps pointing the same way in space while the vehicle rotates around it. Accelerometers on the platform measure every push, and a computer adds those pushes up, over and over, to track speed and position.
An everyday example is the motion sensor in a phone, which notices when you turn it sideways. Apollo's platform hung inside three nested rings called gimbals, so the craft could turn freely around it.
Three gimbals have a weak spot: gimbal lock. If two rings line up, one direction of rotation is lost and the motors cannot keep up. The Apollo lunar module's computer warned at 70 degrees on the middle gimbal and froze the platform at 85; on Apollo 12 a lightning strike faked an 85-degree reading, and the crew realigned the platform by sighting stars.
A common misconception is that inertial systems never go wrong. Small errors add up, so the platform drifts and has to be corrected from time to time against the stars or other references.
