A giant loop of air along the equator: air rises over the warm western Pacific, flows east high up, sinks over the cool eastern Pacific and returns west as the trade winds.
El Nino basics
Picture a conveyor belt of air over the Pacific:
Over the very warm water near Indonesia (the Western Pacific warm pool), air heats, rises and forms big rain clouds.
High in the sky, that air flows east.
Over the cooler eastern Pacific, the air sinks. Sinking air is dry, so skies are clear there.
Near the surface, the air flows back west. That surface flow is the trade winds.
During El Nino the warm pool moves east, and the rising branch moves with it. The loop weakens. Over Indonesia and nearby, air starts to sink instead of rise, which means fewer rain clouds.
Why it matters for Koh Samui
Sinking air over Southeast Asia is the main physical reason El Nino tends to bring drier, sunnier and hotter weather to Thailand.