From the Manufacturer SARIS - For a stable, worry-free ride with any rear wheel-mounted trainer. Unique 3-level design lets you select the level of difficulty. 
This riser accommodates multiple wheel sizes and widths and allows you to level your bike with the trainer or slightly raise the front of your bicycle to simulate a climb. 

This item is new and unused; it was split from an accessory kit that included a mat and towel (9781T).