The paint bucket tool will fill an area with your chosen color. The default shortcut key for this is G.

On the right, underneath your color wheel, you’ll find a range of settings and behaviors for your paint bucket.

Target layer:

This setting determines which layer’s contents will inform your fill.

Active: fill only according to the contents of your current layer

All: fill according the content of all layers

Above: fill area according the contents of the layer above your active layer

Below: fill area according the contents of the layer below your active layer

Mode:

This setting determines what characteristics on your selected layer will inform the fill.

Color: this will create the fill on an area of the same color.

Opacity: this will create a fill on an area of the same opacity.

Smart: this will fill to the edges of a line or transparent area.

Fill Mode:

This setting gives you some options for the behavior of the Fill tool.

Fill: standard fill based on either color or opacity

Fill rest: an content outside of the shape of color you choose will fill

Clear: instead of filling an area with color, it will erase that area instead.

Clear rest: erase the contents outside a specified area