Grounding

Grounding gives every signal in a system a common reference 'sea level' to be measured against — because a voltage means nothing until you say 'compared to what?'. Just as map heights only make sense relative to sea level, every 3.3 V or 5 V in your circuit is really '3.3 V above ground'. Ground is also the return road every current must travel back along to complete its loop, and a safety path that whisks a dangerous fault current away from a human and into the earth.

In EMC, grounding is where good intentions go wrong, because real ground is not a perfect zero-resistance plane — it has resistance and inductance, so currents flowing through it create small voltage differences between points that were 'supposed' to be the same. Those differences are noise. The art is to control where return currents flow: keep them on a solid ground plane, give noisy power circuits and quiet signal circuits their own clean return, and join grounds at a single point so stray current can't wander through sensitive nodes.