A mixed media painting is one which combines different painting and drawing materials and methods, rather than only one medium. Any materials can be used, including collage items such as pages from magazines, newspaper, photographs, fabric, soil, or packaging. Or a mixed media piece can be as 'simple' as using two mediums, such as acrylic paints with pastel on top.
Mixed media isn't a 20th-century phenomenon, although in previous centuries artists were less experimental in what they used. For example, gold leaf was often added to church paintings; Leonardo da Vinci mixed pastels with other drawing media; William Blake used watercolor washes to his prints; Edgar Degas combined pastels with charcoal and printing inks.