Spree

Spree is a transparent layer to which most things you draw(or erase) goes first, before being solidified into a selected layer. This means, by using spree controls, you can retouch things you put unto a layer, beyond just undo.

Spree feature has two modes: draw and erase.
Toggle spree mode by pressing X key
or clicking Spree indicator on the left toolbar.

e.g. when using Brush Tool you can draw in draw mode, and then erase parts of what you just drew in erase mode. Leaving layer's previous contents untouched.
Similar is available for Eraser Tool too, spree erase mode will be undoing erasing, bringing original content of a layer back.

note: layer Mask does not use spree; spree is active only when drawing to a layer.

Solidify / Discard / Auto
Top panel features a spree options tab.
Auto option automatically draws contents of spree to selected layer (solidify spree) when you pick a new color.
Solidify spree button allows to manually solidify spree.
hotkey: Shift+S
Discard spree button discards contents of a spree layer

Usage
Spree indicator shows which spree mode is currently active draw / erase.
More information about Spree indicator here
Tools - many tools support and utilize Spree feature.
Check with each tool for more information.
As an example: Fill Tool, when filling to spree first - can then toggle spree mode erase, and it will change the tool to Brush and you will be able to retouch the filled region.

note on Fill Tool: to fill directly to a layer / color replace - set 'Direct' option in tool options menu.

Selection from Spree
Spree can be used to make a selection. Turn spree into a selection by:
Press Ctrl+B
or Select->Spree to selection

Undo Solidify If spree has just solidified and you would like to get back to the state before -
Use Undo and then a single Redo.

for Undo: press Ctrl+Z keys or Edit->Undo
for Redo: press Ctrl+Shift+Z keys or Edit->Redo