Just quoting Wiki since it's quite succinct and accurate on this: "[The Wolf Amendment] prohibits the National Aeronautics and Space Administration (NASA) from using government funds to engage in direct, bilateral cooperation with the Chinese government and China-affiliated organizations from its activities without explicit authorization from the Federal Bureau of Investigation and Congress."
For another consequence of this law, when China relatively recently carried out a sample return from the Moon, they sought to share the resultant rocks/material with countries worldwide, much like NASA did in the 60s. Except Americans couldn't accept them, at least not without jumping through a million hoops first, due to this law. It's one of the ever more frequent 'I'm going to punch myself in the face because I don't like you' acts by governments.