The translation manipulator
To adjust an object's location, click the translation manipulator button () in the lower-left corner of the viewport. You can drag the arrows that appear at the axes of the selected object, drag the circles to move the object along a specific plane, or drag the blue square at the center of the arrows to move the object in screen space.
When you're dragging the blue square in the middle, you can also use one of three modifiers. Holding downwhile you drag moves the object along the world's surfaces. Holding down while you drag snaps the object to nearby joint controls. Holding down while you drag moves the object closer or farther away.