To scan a line is to mark which syllables the voice leans on. English keeps a quiet pulse underneath — weak STRONG, weak STRONG — and a pair of syllables is a foot. The weak–STRONG foot is the iamb, and five of them is iambic pentameter, the heartbeat of Shakespeare and blank verse.
Meter isn't the poem — it's the grid the poem plays against. Reverse a foot (STRONG–weak, a trochee) or double the stress (a spondee) and the line pushes back on its own pulse. That friction is where the meaning lands.
Robert Frost called it the sound of sense: his power comes not from keeping the beat but from the moment he breaks it — “Something there is that doesn't love a wall.” The reversed first foot throws its weight onto Something.
Break too many feet, though, and the ear simply hears a new regular meter — the tension needs the old grid still felt beneath it. Tune the pulse below and listen.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.