It's a query that gives you a list of issues matching some criteria, at the time that you run it.
So you can answer questions like "show me the issues in Project X" or "show me the issues assigned to Dave", or "show me the issues updated in the last seven days".
You can save filters and reuse them in reports and gadgets.
You might check here;
https://confluence.atlassian.com/jira064/using-filters-720416526.html
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