Occasionally, adjacent opposing groups will become surrounded while neither group has two eyes. If one group has enough extra liberties, it may be able to capture the other. Otherwise, a seki, or dual life, is a likely result of the battle.
In the position at left, the White group in the lower left has two eyes, and so is safe. The two Black groups in the corners at the top have just one eye each (the corner points), but are safe because White cannot play on either point marked X without allowing Black to capture the White stones. Meanwhile, the five White stones on the right and the large Black group at the bottom are both eyeless, but both groups are safe because neither player can occupy either of the points marked Y without allowing the opponent to play on the other point Y and capture them. Since neither player can approach the other anywhere, the various groups with fewer than two eyes are alive in seki.