Skin-tight clothing is clothing that is so tight that it clings to the wearer's skin. It is normally made of stretchy cloth containing substantial amounts of Lycra, allowing it to take the shape of the wearer's body. Latex clothing may also be skin-tight.
Another way to make clothing really tight is to put it on and get it to shrink. Clothing made of wet rawhide will do this. At one time, there was a fashion to put on cotton jeans that would shrink, and sit in a bath of hot water; such shrinkage was often only temporary. Modern jeans are often pre-shrunk so this no longer works.
Skin-tight clothing may be slightly see-through because the material is stretched. This may particularly happen over the bottom when someone bends over or climbs stairs, rendering the panties (or lack of them) visible.