Complex sentence: Agde is the Mediterranean port of the Canal du Midi, which connects to the Hérault at the lock ("L'Écluse Ronde d'Agde") just above Agde and debouches into the Mediterranean at Le Grau d'Agde.
Split into simpler sentences: Agde is the commune in the Hérault department in southern France that is the Mediterranean port of the Canal du Midi. The Canal du Midi connects to the Hérault at the lock ("L'Écluse Ronde d'Agde") just above Agde and empties into the Mediterranean at Le Grau d'Agde.