Beginners count only touching stones. Strong players count lines: two stones on the same row are a weapon even with four empty points between them, because one link point turns them into a four-in-waiting. Here black owns row 7 at points 4 and 8. Playing 6 (or 7) is the link: after it, the companion point completes five — the whole row is one move from winning. White sees the row too, which is exactly why the link must be played before white builds on it. Given one free move, white kills the whole row for a single stone — long-range weapons are fragile precisely until the link is paid for.
Finish this step to move on.