Most cloth is weaved, meaning it has thousands of small "holes" in between the strands of fabric. If a powerful light source is aimed directly into the fabric, the light will travel through these holes, strike whatever is behind them and bounce back into the lens. If the area behind the holes is white, it will bounce white back, if it is skin, the color of the skin will bounce back. The contrast between the two allows the viewer to "see" the undergarmet.
Also some blouses (especially white ones) are rather see through all the time, regardless of the lighting.
Benji