The Modes represent different editing modes the Level Editor can be put into, enabling specialized editing interfaces and workflows for editing particular types of Actors or geometry.
Activates Place mode for placing Actors in your scene.
Shift + 1
Toggles Paint mode for painting vertex colors and textures on Static Mesh Actors directly in the viewport.
Shift + 2
Toggles Landscape mode for editing Landscape terrains.
Shift + 3
Toggles Foliage mode for painting instanced foliage.
Shift + 4
Toggles Geometry Editing mode for modifying Brushes to geometry.
Shift + 5