Usually pads work like this. On top there is a special layer, that takes liquid away from your skin and pulls it inside the pad. Under the top layer, inside the pad, ultra pads have millions of granules, that turn your flow into gel. Gel is not a liquid, so it can not get away from inside the pad and cause stains. Maxi pads instead of granules have special cotton material to absorb your flow. The last layer is a plastic bottom layer. It stops liquid from leaking through on your panties and causing stains. It also has glue stripes that fix the pad on your panties.
Attribution: P&G/Always