Hi, and thank you for your question.
The star option in Thunderbird is simply one of its multiple "tag" options, and is generally used to mark emails that are specifically important to the user. You can use stars for your own convenience, e.g. to mark emails for reply at a future date, to keep them separate from unimportant emails or to group together emails related to a particular topic or sender.
As an example, to group together starred emails across your entire list of folders, you may click on the Quick Filter toolbar at the top and then click on the star button on the yellow toolbar that shows up below, to only show messages that have stars applied to them. You can also automatically add a star to particular types of messages - choose "Tools" then "Message Filters" to create a new filter. Any of your filters (messages from a particular sender, for example) can have the "Add Star" action applied to them.
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