input resistance
Input resistance describes how thirsty the input of a stage is, how much current it draws from whatever feeds it for a given input voltage. A high input resistance only sips, taking almost no current; a low input resistance gulps, demanding a lot. It is the headline number that tells you whether connecting this stage to a previous one will load that previous stage or leave it undisturbed.
Formally it is just Ohm's law at the input terminals: Rin = Vin / Iin, the input voltage divided by the current the input draws. A good voltmeter has an input resistance of around 10 Mohm, so when it taps even a fairly high-resistance node it pulls almost no current and barely disturbs it. An op-amp input is higher still, often gigohms or more. By contrast, the input of a simple loudspeaker circuit might be only a few ohms.
The whole game of connecting circuit stages is comparing input resistance against the source resistance feeding it. You want the input resistance of a stage to be much larger than the output resistance of the stage driving it, a factor of ten or more, so that almost the full signal voltage transfers rather than being lost to loading. This is why buffers and voltage followers, which present a huge input resistance, are inserted to protect a weak, high-resistance source from a hungry load.
A sensor with 100 kohm of source resistance feeds an amplifier. If the amplifier's input resistance is 1 Mohm, about 1000/(1000 + 100) = 91 percent of the signal gets through; raise the input to 10 Mohm and over 99 percent transfers.
High input resistance avoids loading the source, so more of the signal voltage survives.
For passing a voltage, high input resistance is what you want; it avoids loading the previous stage. A voltmeter is built to have as high an input resistance as possible for exactly this reason.