Action Groups are a way for you to store a common set of actions together that you want to use across multiple commands.
Previously Action Group commands had a special option known as "Run One Randomly", which allowed a single action to be run at random. This option has been removed and instead replaced by the new Random Action which can be used in any command much more easily.
For example, let's say you have a chat command and a timer command that both send a message to chat with a link to your Twitter account. You can make an Action Group which has the Chat Action with your Twitter link, then use that Action Group in both your chat command & timer commands by adding an Command Action to both commands.