Don't know where to start? Don't worry. You can read a quickstart guide in the GitHub Repo You can press the Work in local directory-button and navigate to a minecraft world you want to work in. Then navigate into the "datapacks" directory and create a new directory for your datapack. Open it in the 'work in local directory' dialog.
(Your minecraft worlds are usually in %appdata%/.minecraft/saves)
Your browser does not support editing files (e.g. your datapack) on your disk directly. I highly recommend using a Chromium-based browser instead. You can still use this tool by pressing the Download datapack as ZIP-button and putting the ZIP into your data datapacks directory.