Why is there a minimum input resistance?
You can read it here:
It's not a multimeter, it's a current probe.
You connect it to a multimeter in order to see its output voltage, which is a measure of the current that you are measuring with the probe.
So as the output voltage rises with \$ 0.1 \, V \$ for every Ampere (\$ A \$) it measures, it also has an output impedance which causes the voltage to decrease if you draw a current from the output.
That's why they advise you to take a multimeter with an input impedance of at least \$ 1 \, M\Omega \$.