With individual throttle bodies (ITBs), each cylinder gets its own butterfly valve regulating air intake, rather than all cylinders sharing one valve. When you apply throttle to an engine with a single throttle body, air has to travel further to reach each cylinder, slowing response somewhat. In an engine with ITBs, atmospheric pressure builds outside each cylinder, so when you open the throttle, air enters instantaneously.
ITBs also help reduce pumping losses at partial throttle. Interestingly, ITBs also get rid of the need to have an intake manifold!