A filing is when a legal document becomes part of the public record. Anyone can read a filing.
As you might imagine, a filing is an example of something being put in a file. You can take part in the physical act of filing by organizing documents in separate files. Or, you can execute a different type of filing by making a legal document public. After a public filing, a document is fair game for anyone to read, including members of the media. Another type of filing is produced from filing a metal. If you are filing away at your iron gate, for example, you will create iron filings. Best not to inhale those!