The Smart Parent Window can be opened from the menu bar of Unity under "Tools > Better UI > Smart Parent".

Background

When creating UIs you usually nest objects a lot. Often you have one parent that acts as a bounding of several children.

However, when the children change their position or size or another child is added, the children might exceed the parent's bounds.

This problem is not only aesthetically. When you work with anchors for controlling sizes (see Snap Anchors) and prefabs, it can become necessary that you have such a relation.

As a workaround you can drag all children out of the parent, resize the parent and drag the childs in again and snap all anchors.

To overcome this annoying workaround, you can use the Smart Parent window. It allows you to resize / move the parent without affecting the screen position of its children.

Options

Select Parent

This is just a shortcut to select the parent of the currently selected object.

Trim

Note that the "Trim" options are only available if you have one object selected which has a RectTransform component and at least one child.

When choosing one of the trim options, the selected object moves / resizes while all children keep their visual position and size.

Trim Horizontally and Vertically

Changes the anchors of the parent so that they exactly cover the children. DeltaSize and AnchoredPosition will be set to 0, so that the anchors define the covered area of the parent.

Trim Horizontally

Works similar as the big button but only affecting the anchors, size & position horizontally.

Trim Vertically

Works similar as Trim Horizontally but for the vertical direction instead of the horizontal direction.

Detached Children Mode

on: / off:

As long as this mode is toggled, the currently selected object can be moved and resized freely while all children keep their visual position / size.

Be Careful: 

When resizing a parent, its size should never become 0 in any direction. This can break the transform information of the childs and undo might not work in such a case! 

Also sizes close to zero can be problematic and can change the visual position of the children slightly.