A single print appears as a lone letter with nothing stacked beside it. It indicates that price passed through the level so fast that only one bracket registered any trade there.
Because there was no two-sided auction at those prices, they are considered unfinished business, and profiles frequently fill them in later. The concept is essentially identical to imbalance and fair-value-gap in other vocabularies, just measured with a different tool.
They are far from guaranteed to fill, and single prints at the start of a strong trend often survive for a long time. Their more reliable use is as a marker of where a move accelerated, which helps identify the origin of an impulse.
Related: imbalance, fair-value-gap, tpo, market-profile, low-volume-node