The atmosphere splits the Sun's white light into separate colors like a prism: red bends more than orange, orange more than yellow, and so on. As red undergoes the strongest curvature, it appears to go below the horizon first, followed by orange, yellow, and green.
The colors after green - cyan, indigo and violet - are strongly scattered by gases in the atmosphere. That's why the sky is blue. And so the last color that can be seen when the Sun goes below the horizon is green.
Usually this effect is very weak. For the last green rays to be visible, a mirage must also appear, making the Sun appear larger than usual. These mirages can also cause the Sun to move in shimmering waves until it is almost liquid and pours over the horizon.
The ocean horizon most often produces the best green light mirages.