Gimbal Lock occurs when the rotation of an object seems to "stick," as it is rotated all the way down or all the way up.
This can also happen with the camera. It happens because the mathematics in rotational systems like Euler angles cannot make a consistent rotation solution when they point straight up or straight down. The Quake games avoid gimbal lock by not allowing the viewer to look straight up or straight down.
To solve gimbal lock, either avoid looking directly down or directly up, or else use a different rotational system.