A shower curtain liner is a waterproof or water-resistant material that hangs inside a shower or bathtub to protect the outer decorative shower curtain from moisture and mildew. Typically made from materials like vinyl, polyester, or PEVA, these liners help keep the bathroom clean and dry by preventing water from splashing outside the shower area. Shower curtain liners come in various sizes and styles, allowing them to fit different shower setups. They are often easy to clean and can be machine-washed or wiped down, making them a practical addition to any bathroom. Many liners also feature weights at the bottom to help them stay in place during use.