Time dispersion is a display of multiple pathways that spread the signal in time so that the length of time of the received signal is greater than the signal that was sent out.
Actually due to multi-path propagation, a transmitted signal received at receiver via different path. So due to this delay, receiver can't receive the signal within the intended time frame. so avoid this situation, usually it is known as "ISI" inter symbol interference. a CP(cyclic prefix) is added at the start of the symbol. CP is nothing but the end portion of a signal which is being transmitted.