Soloing temporarily hides all layers apart from the selected layer or layer group. This allows you to focus on editing the selection without any distraction.
Solo mode lets you continue to edit on the currently selected pixel layer, e.g. paint, erase, transform, etc. If you perform any operation that creates a new layer (e.g., create an empty pixel layer, apply an adjustment layer or adding a curve), the operation will automatically exit the selected layer from solo mode.
To exit the mode manually, tap Solo again.