As of Unreal Engine 4.24, you can no longer access the Matinee Editor from the Cinematics Menu and Details Panel. This is part of the transition to remove Matinee from UE4 completely in 4.25. You can still convert Matinee actors to Sequencer assets using the Matinee to Sequencer Conversion Tool.
The Matinee animation tool provides the ability to animate the properties of Actors over time, to create either dynamic gameplay or cinematic in-game sequences. The system is based on the use of specialized animation tracks in which you can place keyframes to set the values of certain properties of the Actors in the level. The Matinee Editor is similar to the non-linear editors used for video editing, making it familiar to video professionals.