Leaching has to do with preferentially dissolving the more highly reactive copper out of the surface of a silver coin, either by natural artificial means. Applies more to silver denarii with a higher proportion of copper in the alloy, especially post Severan. In the natural situation, a denarius can become more porous and fragile, and so should be handled gently.
With modern .500 fine silver coins, the blanks are rinsed with acid to remove some of the copper out of the surface. (known as 'blanching'). The blanks are then double rinsed with water, and dried before being formed into planchets and then struck into coins. This process enriches the silver in the surface layer of the coin, to well above .500 fine.
Even the Incas used a similar method to enrich the gold in the surface layer of their gold artwork. Earlier, they discovered that a gold / copper alloy was much stronger.