Hinge joints allow bones to move in one plane in relationship to each other.
Outside of the human body, think of a door. As it hangs on the hinges holding it in place, it can only move back and forth. It is always in the same alignment with the door frame, it is always at right angles with the floor, it is always in the same vertical orientation as the wall.
In the human body, the joints in fingers, toes, elbows, and knees are hinge joints. The bones in any of those joints move along one plane in relationship to each other. Unlike the door example, however, the orientation of the bone closest to the body trunk may change orientation and make it appear that the joint is moving in other directions. You can rotate the position of your upper arm, which allows your elbow joint to move your lower arm into different positions relation to your trunk, but the lower arm is moving in the same plane in relation to the upper arm.
The range movement allowed by a hinge joint," is limited to one plane." Imagine opening and closing a door. The door is limited to being opened and closed. A commonly known hinge joint is the elbow. This is where the humerus (or funny bone) meets the ulna. Bend your elbow. It only can bring the forearm to the upper arm and back straight again. Therefore, this hinge joint only moves on one plane.