This is a multi-page tutorial about creating ground pour on an Eagle layout. The picture above shows an example. Note that the ground copper fills in space underneath resistors R1, R2, R4, and R6. It also runs up under the stack of components on the right and in between the +9V trace and C1.
First, I have a short editorial. Many people make a distinction between ground pour and a ground plane. What you see above is ground pour. In PCB fabrication, a ground plane is a layer of copper that is distinct from the layers for signal and power traces. Quoting the wikipedia article Ground plane,
A ground plane in PCB assembly is a layer of copper that appears to most signals as an infinite ground potential. This helps reduce noise and helps ensure that all integrated circuits within a system compare different signals’ voltages to the same potential.
It also serves to make the circuit design easier, allowing the designer to ground anything without having to run multiple tracks; the component needing grounding is routed directly to the ground plane on another layer.
Ground planes can also be placed on adjacent layers to power planes creating a large parallel plate capacitor that helps filter the power supply.
One more thing before we start. You need to have my Eagle environment for this tutorial. If you are following along with your version of Eagle, type in each of the following six commands:
grid mil 50 2
ch lay bot
ch wid 40
ch pour solid
ch the off
ch orph off
ch iso 40
set polygon_rat on
These set the units of measurement, make sure we are working on the right layer (bottom), set the trace width to 40mils, and some other stuff related to the Eagle POLYGON command that makes the ground pour.