The rest is bits of the unused lining and other fluids. By the time your period ends, it will have amounted to between half a cup and a full cup of liquid. Most women use either sanitary pads or tampons to absorb the flow. You can buy them in drugstores or supermarkets. Usually they come in packages of ten or more.
Every package has instructions in it.
They come in different sizes and varieties. Some are for lighter flows. Some are for heavier flows. You will need to decide which type of pad or tampon is most comfortable for you. Most pads stay in place by sticking to the inside of your panties.