Basically taken, the Reason Refills are soundbanks, that can contain any of the Reason's patch, an audio-file or a songfile save. Anyone with a legit copy of Reason can download a Refill Packer and make a Refill. This is a great way for to the sound-designers to make patches and pack their work inside a Refill. The packed content will be compressed and when a user loads a sound, it will be uncompressed and will be in a full quality.
The Reason's soundbrowser makes it possible to explore, that what kind of sounds there are inside the Refills. You can also search content by a name - as example typing a 'Thunder' and pressing enter. Very handy to find someting intentional.
You can download the Refill Packer HERE.
The Reason Studios Refills
google.com, pub-0445311970748285, DIRECT, f08c47fec0942fa0