General

Allow editing on non-visible layers

You can edit layers with disabled visibility (the eye icon in the layer dock).

Save Tool Options On Exit

Self-explanatory

Save Tool Options Now

Self-explanatory

Reset Saved Tool Options To Default Values

Self-explanatory

Scaling

Default interpolation

When you scale something, each pixel in the result is calculated by interpolating several pixels in the source. This option determines the default interpolation method: it can always be changed, though, in the Tool Options dialog.

There are five choices:

None

This is the fastest method, but it's quite crude: you should only consider using it if your machine is very seriously speed-impaired, or when you are designing pixelized images.

Linear

A simple form of interpolation which is good enough for most purposes.

Cubic

This is the default choice, but also slower than Linear. In general this gives better results than Linear, but there are some cases where it can actually look worse.

NoHalo

This method performs a high quality interpolation. Use the NoHalo method when you downscale an image to less than a half of the original size.

LoHalo

This method performs a high quality interpolation. Use the LoHalo method when you do not reduce the size much (rotate, shear).

Paint Options Shared Between Tools

Brush, Dynamics, Pattern, Gradient

You can decide here whether changing the brush etc for one tool should cause the new item to be used for all tools, or whether each individual tool (pencil, paintbrush, airbrush, etc) should remember the item that was last used for it specifically.

Move tool

Set layer or path as active

You can decide here whether changing the current layer or path when using the move tool and without pressing any key.