Beacon looks at your saved servers list, and if there are any outdated or missing mods in your Minecraft installation, it will show them in a list. You can then download them straight from the in-game GUI. A quick restart, and you’re ready to play!
Note: Beacon dynamically downloads the latest mod list from GitHub. Updates will not contain new mod lists, they will be updates to newer versions of Minecraft and/or updates to Beacon itself.