Why pass a query parameter to an image file (/img/button.png?123456789) from inside a HTML page?

I saw this in a HTML template for a web app:

<img alt="mybutton" src="/img/button.png?123456789" />

What I didn't understand was why the ?123456789 would be passed to the image file? (in the开发者_如何学Python actual app, the number that was actually passed seemed to be customized to the user's session or unique id perhaps)

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It forces the browser to skip its cache, and retrieve a fresh copy from the server.