Every surfactant molecule has two different ends. One end is a long chain that avoids water and is called hydrophobic. The other end likes water and is called hydrophilic.
At a boundary between water and air, or water and oil, surfactant molecules line up. Their water-avoiding tails point away from the water, and their water-loving heads point into it. This arrangement lowers the surface tension sharply.
When enough surfactant is added, the molecules start to clump into tiny balls called micelles. Inside a micelle, the tails cluster together in the middle, away from water, while the heads face outward into the surrounding water. This structure lets a micelle trap oil or grease inside its core, which is exactly how soap lifts grease off your skin or dishes.
Below a certain concentration, called the critical micelle concentration, surfactant molecules mostly stay spread out at the surface rather than clumping into micelles. Not all surfactants come from petroleum. Many newer formulas use biosurfactants, made by fermenting plant sugars with microbes, since these break down more easily in the environment.
Surfactants also help spread pesticide sprays evenly across leaves and keep paint pigments mixed rather than settling out. Surfactants show up in soap, shampoo, laundry detergent, and countless industrial products where oil and water need to mix or separate cleanly.