What is an eye
An eye is an empty point whose four orthogonal neighbours all belong to one connected group — diagonals do not affect the definition, though they will decide whether the eye is real (8.2). Eyes matter because of the suicide rule you just met: the opponent can never play inside a true eye, so a group holding two separate eyes can never be captured at all (1.8). This black ring wraps exactly one true eye — a point White could only enter by destroying itself. Black to play: click the eye.