The Timeline displays all the frames in the Flipbook animation.
You can increase the duration of any sprite in the Timeline by draging on the handle of the sprite to increase/decrease its duration.
Mousing over a sprite in the Timeline will also give you more information about the sprite.
Above the name of the Sprite that is being used, its Index (location in the sequence) and Duration in frames are displayed.
You can also Right-click on a key frame to bring up a context menu with additional key frame actions.
From the context menu, you can Duplicate or Delete the key frame or you can Insert Key Frames before or after the selected key.
When you insert a key frame before or after a key, an empty key frame will be added (indicated in black below).
You will need to specify the sprite to be used for those key frames, otherwise the Flipbook will render nothing in those frames.
Sprites or Flipbook assets can also be drag-dropped onto the Timeline to append them as additional key frames to the end of the Timeline.