Operation amplifier concept problem
It isn't clear, from your question, what concept you're having a problem with.

According to the schematic, we can write the following equations

$$v_+ = v_{IN}$$

$$v_- = v_{OUT}$$

We also have, by definition of the op-amp with gain \$A\$

$$v_{OUT} = A(v_+ - v_-)$$

(note:  \$v_+\$ and \$v_-\$ are voltages at the non-inverting and inverting inputs respectively)

Combining these equations yields

$$v_{OUT} = A(v_{IN} - v_{OUT})$$

Solve for \$v_{OUT}\$

$$v_{OUT} = \frac{A}{1 + A}v_{IN}$$

Thus

$$v_+ - v_- = \frac{1}{1 + A}v_{IN} $$

For arbitrarily large \$A\$ (or taking the limit as \$A \rightarrow \infty\$), we have

$$v_{OUT} = v_{IN}$$

$$v_+ - v_- = 0$$

Now, is there anything in the above that you're having a problem with?