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.
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
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.
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