Activated charcoal/carbon is basically just a structural carbon that is highly porous and sponge-like. It has a massive surface area for its volume and can trap other materials and act like a filter. It can remove smells and organic molecules that can make the water taste funny. You can easily make your own activated charcoal from a variety of materials and in pretty large quantities. There are plenty of videos on YT that show the process, but here's one from [Cody's Lab](https://www.youtube.com/watch?v=GNKeps6pIao) to get you started if you want to do this.