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Nothing shows up when I try it or I get an error that file_get_contents doesn't exist

This script relies on the function file_get_contents to fetch the RSS feed. If your host doesn't allow the use of it, but allows the use of curl, you can replace

$stories = file_get_contents($uri);

with

$ch = curl_init();
$timeout = 5; // set to zero for no timeout
curl_setopt ($ch, CURLOPT_URL, $uri);
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt ($ch, CURLOPT_CONNECTTIMEOUT, $timeout);
$stories = curl_exec($ch);
curl_close($ch);

What does cache time mean?

It's the number of seconds before the content will be requested again. You can set this to be 0 or -1 to not use the caching system. If you edit any of the widgets in the widget admin panel, the cache is cleared.