the ground plane

Instead of threading thin individual ground wires all over a board, you can devote an entire layer of solid copper to ground, one continuous sheet sitting under the whole circuit. That sheet is the ground plane. Think of it as a calm, wide lake that every part can dip into right beneath itself, rather than a tangle of narrow canals: there is always a short, low-resistance path to ground directly below any component, wherever it sits.

A solid plane works for two deep reasons. First, its resistance and especially its inductance are tiny, because current can spread out across a wide sheet rather than crowd into a thin trace, so ground stays at very nearly the same voltage everywhere. Second, and more subtly, a high-frequency return current does not wander off randomly; it flows in the plane directly underneath its own signal trace, because that path encloses the smallest loop and so has the lowest inductance. This keeps each signal-and-return loop tiny, which both lowers radiated emissions and makes the board far less susceptible to picking up interference.

This is why almost every fast or sensitive board uses at least one full ground-plane layer; it is the high-frequency answer that pure star grounding cannot give. The crucial practical rule is: do not cut slots or gaps in the plane under a signal trace, because that forces the return current to detour around the gap, enlarging the loop and creating exactly the noise and radiation you were trying to avoid. Honest note: a ground plane is a layout choice the schematic cannot show, and it is one of the clearest cases where the board itself IS a circuit element.