What it all means is that a high-impedance circuit placed in parallel with a device under test has no more effect upon the D.U.T. than a voltmeter (which is a high-impedance circuit placed in parallel with a device under test).
[There is, in fact, (to split hairs) some negligible effect from that because it's a high or very high impedance, but not an infinite impedance.]
Back in the bad old days, voltmeters weren't the high-impedance wonders that they are today.