When two things are put together to form a joint, they need to have something in between them to make sure that the seal is air and water tight. That thing is a gasket. One example of where gaskets show up is in your car engine. The pistons in the engine have a gasket around them, so that they form a tight seal in the cylinder, so that they fire properly.
But, those aren't the only place that gaskets can show up. They show up everywhere from canning bottles to airplanes. Custom made gaskets get used in many of those places.
Gaskets are made out of many different materials. They include things rubber, metal, fiberglass, asbestos, or paper. The only requirement is that the material must be flat. If there is any kind of texture to the custom gasket, it will cause the seal to fail.
Custom made gaskets, made out of whatever material necessary, will make it possible to any kind of joint, even oddly shaped ones, to get that air tight, water tight seal.
There are so many different places where gaskets get used. There are also many kinds of materials that gaskets can be made of, making them meet any and all requirements.