The idea of OFDM is to spread data across multiple overlapping subcarriers. As they are orthogonal it is still possible to demodulate them. That means, all other subcarriers are 0 at the evaluation point of a specific subcarrier.
Orthogonality is achieved by generating a rectangular subcarrier pulse. By applying a IFFT a $\sin(x)/x$ emerges which allows orthogonality.
Another requirement for orthogonality concerns the subcarrier spacing. It must be equal to the reciprocal of the symbol period. Howver, I do not understand why. I thought it is most important to create these $\sin(x)/x$ that are shifted across the x axis on an unique position for each subcarrier.
Question: Why would orthogonality break if the subcarrier spacing was not reciprocal of the symbol period?