The Float command converts a normal selection into a “floating selection”. You can find more information about floating selections in the Glossary.

In early versions of GIMP, floating selections were used for performing operations on a limited part of an image. You can do that more easily now with layers, but you can still use this way of working with images. In addition to using the Float command, you can also create a floating selection by moving a selection using the “Move” tool, without pressing the Alt key (or Shift-Alt in Linux), which automatically creates a floating selection. You can also create a temporary layer, named “Floating Selection”, with the entries on the Layer menu.

You cannot perform any operations on other layers if the image has a floating selection, so after you make the changes you want to the floating selection, you have to anchor it. That is, you have to attach it to a normal (non-floating) layer, usually the original layer (the one which was active previously). To anchor the floating selection, use the Anchor Layer command. You can also anchor it to an existing layer by clicking anywhere on the image outside of the floating selection, which then merges it with the background layer. You can also anchor it to a new layer by using the New Layer command.