What is loading effect of op amp?

An 'ideal' op-amp has infinite input impedance, and real ones that you can buy for a
nickel come pretty close. That means that the input impedance or "loading effect" of
an op-amp all by itself is nearly zero, and the loading effect of a circuit that you
build with an op-amp is completely determined by the components you choose to
connect to it. You design the circuit to load the previous stage in any way you want.