OFDM subcarriers are packed relatively tightly together. If you look at the original OFDM signal in the frequency domain, you may wonder why adjacent subcarriers are not interfering with each other. The answer is that subcarriers are orthogonal to each other. Even adjacent subcarriers, have 0 influence on each other, and are independent in that sense.
It can be proven that OFDM is the most efficient way to pack these subcarriers together in the sense that it is the smallest inter-subcarrier spacing whereby the subcarriers can all be orthogonal to each other.
So then, in the real world, during transmission, things may happen besides getting white noise added, that cause the subcarriers to lose orthogonality to each other, and become correlated at the receiver. Probably your textbook would explain what may cause the subcarriers to get correlated?
In that case, there would be inter-subcarrier interference (also known as inter-carrier interference), and that would give you some non-zero BER.