In autonomous applications such as robotics or vehicles using a camera to track where the camera are is in space can be a good localization estimate. The idea is to analyze sequential frames in a video and compare the movement of certain features in the image between them. Once the correlations are found either a Kalman filter or a 3D mapping can be found depending on the type of data available. Visual odometry can be a highly accurate measure of position but it's often used in sync with other localization and mapping techniques.