Something that is permeable can be passed through, especially by liquids or gases. "I wish I hadn't worn my permeable sweater to the picnic when the weatherman called for thunderstorms. The rain seeped right through the fabric, soaking me to the skin."
A permeable surface allows materials like liquids to pass through — either in or out. Inside the body, the walls of cells are permeable membranes that allow fluids and nutrients to get in and nourish the cells. A permeable shirt is good to wear in the summer, because it helps release the sweat sticking to the skin.