There are four tools that can create or modify selection:

Rectangle Select Tool, Ellipse Select Tool, Lasso Tool, Magic Wand Tool.

Selection is to protect image when you draw on the image. The outside of the selection will not be affected.

The following is a sample. First I use Ellipse Select Tool to draw a selection. Then I use Fill Color to fill the selection with black color.  You can see the color only filled into the area of the selection.