Every PC board has some point or points that are solid, minimally noisy grounds,. These points are often called ground points, solid ground, reference ground, or any number of similar names. The reference ground is usually where the power source's ground physically connects to the board. If the power source is on the board, the reference ground is the physical connection of the output capacitor for the source. It is very important to realize that from a noised minimization perspective, every other ground location/point on the board is not the same as reference ground and is usually noisier (never less noisy) than the reference ground. Even if you have a massive ground trace from one side of the board to the other there could be noise generated along that trace with respect to the reference ground.
Remember that traces are typically thin copper conductors and have a small amount of resistance. When current flows through the trace, there is a voltage drop across the trace due to that resistance. So the ground on the other side of the board is not electrically the same as the reference ground. Why is this important? Because all signals entering or leaving the board are referenced to the reference ground not to the ground of that particular circuit. So any ground noise generated on the board will be seen by the off board circuitry.
So for example, consider a thick trace from point "A" to point "B" and call it trace "A-B". Point "A" is the reference ground and point "B" is the grounding point for a high speed switching circuit. There is a lot of transient ground current flowing between point "A" and "B" and a significant voltage drop occurs along "A-B" during these transients. Now introduce a sensitive high gain amplifier on the board. During PCB layout, the common practice to minimize noise is to minimize trace length by connecting to the nearest ground point. In this case the nearest ground point to the amplifer is trace "A-B". "Don't Make that Connection"! If you connect the ground trace of the amplifier to trace "A-B", the ground reference for your amplifier is going to be very noise resulting in a very noisy output signal from the amplifier.
A better, quieter layout would result by routing a longer trace from the ground connection for the amplifier all the way back to the reference ground point "A". With this routing method, the amplifier's ground reference is quiet, maximizing its performance.
Another point to remember is that the more ground current present, the more voltage offset and/or voltage noise is created. Using ground planes and/or separate ground layers helps reduce these ground voltages, but avoid narrow channels or passages in the plane/layer connecting to the reference ground. Otherwise, the same type of noise issues can appear.