growth is temporarily slowed or interrupted at the nail matrix—the area beneath the skin at the base of the nail where the nail is produced.
Think of the nail as a timeline.
Something happens to the body → nail growth is temporarily disrupted → the nail continues growing → the previous interruption eventually becomes visible as a horizontal groove.
Because fingernails grow slowly, the position of a line can sometimes provide an approximate clue about when the underlying event occurred.
However, the line itself doesn’t tell you exactly what caused it.
