The term "multipath" describes a situation in which a transmitted signal follows several propagation paths from a transmitter to a receiver. This may result from the signal reflecting off several objects to arrive at the receiver
This is a problem with indoor systems because office environments are typically congested with metallic furnishings like filing cabinets. With multipath, the signal path lengths are unequal, which results in several time delayed copies of the information signal arriving at the receiver. The received signal is then the sum of all signals according to their phase relationships. Severe multipath results in received signals that are spread out in the time domain. This is referred to as delay spread and causes frequency selective fading. A worst case scenario would be wideband multipath where the received signal is spread so much that the receiver detects the spread parts of the signal as phase transitions. In phase shift modulation, this will cause bit errors.
Spread spectrum modulation helps to combat this specific problem by the correlation process. That is, a phase shifted version of a received signal will be decorrelated after multiplication by the "in phase" PN code. However, if the multipath is inconsistent (which may be the case with mobile users) the problem may be more pronounced and the information may need to be re-transmitted.