They're referring to microshock. If the patient has open sores, lacarations, or otherwise any small or large break in the skin, the smallest voltages on those openings can deliver a hazardous shock due to the human body's high conductance.
For that reason, the bioamplifiers need to have an extremely high input resistance; typically in the order of millions or billions of ohms. And the probes can't drop a voltage over a few microvolts. There was a good link to a company that specialized in these types of amps, and I'll post it up in a few minutes if I can find it.