# Having a hard time understanding this equation in monocular EKF SLAM

Reading this paper on visual odometry, where they have used a bearing vector to parameterize the features. I am having a hard time understanding what the state propagation equation for the bearing vector term means :

The vector N is not mentioned in the equations, so its not very clear what it does. Would really appreciate if someone would help me understand it :)

[...] where $N^T (\mu)$ linearly projects a 3D vector onto the 2D tangent space around the bearing vector $\mu$ [...]