At the daytime both sea and land get heated. At night both of them cooled. But at night the land cools much quickly than the sea. So at night the sea is bit hotter. So the air over the sea surface is get heated. When it get heated density of air gets lower and they will go up to the sky. So there will be a air reduction or void near the top of sea surface. So the air from the land blows towards sea to fill this void. This is how breeze is flowing from land to sea.
Simply stated, a air moves from an area of high concentration to an area of low concentration. When air is unevenly heated, it leads to differently concentrated air. The warmer air is rising, leaving an area of low air concentration. This is what causes the breeze to move.
A sea-breeze (or onshore breeze) is a wind from the sea that develops over land near coasts. It is formed by increasing temperature differences between the land and water; these create a pressure minimum over the land due to its relative warmth, and forces higher pressure, cooler air from the sea to move inland. Generally, air temperature gets cooler relative to nearby locations as one moves closer to a large body of water.