Trapped air.

It's hard to believe, but it's true. The man in the photograph is kept warm by the holes in his string vest! The air in the holes is what matters. When the man pulls on his shirt, it traps air in the holes in the vest. The air is an excellent insulator. It prevents his body heat from escaping and so keeps him warm.

A string vest may seem to be a rather strange piece of clothing, but, in fact, aII of your clothes keep you warm in the same way. Clothes have tiny pockets of air trapped between their fibers and so act as insulators.
This insulation is important as you have a heat-loss problem. Your body is warmer than the air around you and so it is always losing heat. The insulation supplied by clothes cuts down the amount of heat which escapes and so saves valuable energy.

For a long time, the warmest clothes were made out of natural materials like wool, fur and feathers. These all have tiny pockets of air happed in them and are excellent insulators. But now man-made fibers are often used instead. A warm anorak is likely to be filled with thousands of fibers made of a plastic like polyester - with air trapped between them.