The Freeform Selection tool allows you to select irregularly shaped portions of your image
– some other programs refer to this as "lasso select".
You may then move, resize, apply effects to, copy, or duplicate the selected region.
A freeform selection is defined by a mask, which specifies the parts of the selection
that show through. You can remove the mask to effectively turn the selection rectangular.
Note that when you apply filters to a masked selection, the mask is retained even though
the filter preview may imply otherwise.
If your selection contains transparency, you can
achieve a similar effect via the alpha channel by setting the Selection Compositing Mode to "Overlay"
via the Format menu.
See also: