The line text (爻辭, yáocí) is the short passage the ancient book attaches to each of the six lines of a hexagram. That's 64 × 6 = 384 texts, plus two extras: the use nine, use six of hexagrams 1 and 2. In all, 386.
How it's labeled
Each text carries a heading that gives the place and the type of line: 初九 ("nine at the beginning"), 六二 ("six in the second place"), 上九 ("nine at the top"). Nine is yang and six is yin, as the line values explain, and the places are counted from the bottom up (see line positions).
What it sounds like
Line texts often follow a story that moves up through the hexagram. In Activity, a dragon appears at each stage: hidden below, seen in the field, leaping over the deep, flying in the sky and, at the top, too high, with regret. Many end with formulas like no fault or regret.
In your reading
Usually you only read the texts of lines that came up as a moving line. They point to the part of the situation that's in motion for you. In Cambios, every line has its practical layer and its poetic layer.
Origins: tradition and scholarship
Tradition credits the line texts to the Duke of Zhou, King Wen's son. Scholars see them as part of the ancient Zhouyi, compiled in the Western Zhou period from divination formulas, with no single proven author. The commentary that explains them is the Small Image.