With Splice, you can layer multiple elements, such as effects, text, photos, and videos. If the elements on the top level of the timeline overlap, you can switch between the elements to select and edit them.
To switch between layers, tap the little marker which appears on top of the selected layer. The icon varies according to the feature chosen, such as a “T” for text, a tiny thumbnail for your photo or video, or a magic wand icon for effects. Tap the marker to cycle between layers.