What do arrows M and N indicate? Also, what does the dotted line AB represent?

If you would ask people to describe the direction of the wave, you will probably get different answers even if there is already an illustration available. For this diagram, the arrow M indicate the incoming wave. The arrow N indicate the reflected wave. The dotted AB line represents what can be considered as normal.

For those who are not aware, waves represent the disturbances that are experienced along a certain medium. It will be useful to get to know the two types of waves to gain further understanding about it. The two types of waves are transverse waves and longitudinal waves.