The primary hexagram (本卦, "root hexagram") is the one you get straight from your cast: the six lines just as they came up, bottom to top. It's the foundation of the whole reading.
What it describes
The primary hexagram is a snapshot of the moment: where you stand in relation to your question, the forces at play and the attitude that suits you. Its judgment and image are where your reading begins.
How it forms
You cast six times and note each value (6, 7, 8 or 9). Odd values are yang lines; even values are yin. That six-line figure is the primary hexagram. Any line that came up as 6 or 9 is a moving line, and within the primary hexagram those are the lines that speak to your specific case.
Alongside the relating hexagram
When there are moving lines, flipping them gives you the relating hexagram. A simple way to read the two together: the primary is where you are, the moving lines are what's in motion, and the relating hexagram is where it's heading.
An example
You cast Initial Obstacles with no moving lines. You're at a hard beginning, with no turn in sight, so read the hexagram and take its advice. If the fifth line had come up as a 9 instead, you'd also read that line's text and then the relating hexagram.
For the full process, see How to Read a Hexagram.