Transform gizmo
The coloured arrows, rings and cubes in an editor that let you move, rotate and scale an object with the mouse.
The convention is nearly universal: red for X, green for Y, blue for Z, an arrow per axis for translation, a ring per axis for rotation and little cubes for scale. Unity and Unreal switch modes with W, E and R, inherited from 3D packages like Maya. Under the hood it is mouse picking: a ray from the camera through the cursor is tested against the handles, and mouse motion is projected onto the chosen axis or plane.
The fiddliest detail is size. A gizmo is drawn at constant screen size regardless of distance, and on top of everything else, or it disappears behind the very wall you are trying to move. It is the most-used tool in any level editor and the least noticed when it works.