"A smaller version of the Portable Ore Advancer that can be attached to the edges of your conveyors for bonus upgrades."
The Ore Stopper is a Rare-tier upgrade that was released in the Summer Update of 2017.
The Portable Ore Stopper shares the upgrade tags to Portable Ore Advancer and Portable Ore Enhancer. Assuming you only use this portable upgrader of the 3, it increases ore value by 14.9% (x1.149 multiplier) up to 31 times.
The Portable Ore Stopper has a shortest upgrader part out of the trio meaning you would need to have your ores on Tiny Conveyors to make sure they get upgraded. The Portable Ore Stopper has the highest multiplier of the trio, making it the most recommended to use in setups.