Use selection tools to select part of the image for example the background only and apply a filter, erase or fill the selected area.

Selection Tools
Rectangular selection
Click and drag a rectangle, hold CTRL to create a square
Elliptical selection
Click and drag an ellipse, hold CTRL to create a circular selection
Polyline selection
Click and release mouse button, move the mouse to a new point and click. Repeat this step for every point of the polyline selection. To close the polyline move the mouse cursor to the starting point
Lasso selection
Click and hold mouse button, draw a free-hand line around an area
Magic Wand
Click inside solid color area to select it. Magic Wand has 1 pixel tolerance so it will select only adjacent pixels of the same color
To erase a selection
Create the selection as above and choose Selection > Erase. Pixels inside the selection will be erased (turned to the background color).
To change the background color
Default background color is white. Click inside the background color to activate it. Change the color with R,G,B sliders or use pipette tool to sample a pixel color.
To fill a selection
Create the selection as above and choose Selection > Fill or press Ctrl+F. Pixels inside the selection will be filled with current foreground color.

Selection Mode
Replace mode
When a new selection is created it will replace the current selection. This is default mode.
Add mode
When a new selection is created it will be added to the current selection (example 1)
Subtract mode
When a new selection is created it will be subtracted from the current selection (example 2)
Intersect mode
When a new selection is created it will be intersected with the current selection (example 3)
Once selection is active, pixels outside the selection will remain unchanged when you use filters (like gamma or brightness), or tools (like eraser, pen or clone)
Inverting Selection
To invert a selection (select pixels outside the selection and deselect pixels inside the selection) choose Selection > Invert
To duplicate a selection
Select an area and choose Selection > Duplicate to copy pixels inside the selection by X and Y offset.
To copy a selection to moveable layer
Select an area and choose Edit > Copy and Edit > Paste. Use Move Tool (white arrow) to move the layer to desired position.

Move Tool
To move a selection path
Choose the Rectangular Selection tool, click on the selection path and drag it to a new position
To move pixels
Create a selection around pixels you want to move, choose the Rectangular Selection tool, hold Ctrl and click inside the selection, drag the area to a new position. To merge the selection with the background click outside the selection area.
Step 1 (preliminary selection)
The first try is not exact, to modify the selection change the mode to Add to Selection
Step 2 (add an area)
To decrease the selection change the selection mode to Subtract from Selection and draw with the lasso tool around the area to be subtracted
Step 3 (remove the area)
Repeat steps 2 and 3 until the selection is perfect
Finished selection