When a topology event occurs and STP converges to a new loop free topology, ports that were previously in a blocking state could potentially be moved to a forwarding state. However these ports can’t simply be turned on to forward traffic immediately otherwise they could also potentially cause loops.
To transition a port from blocking to forwarding and also prevent loops requires that the port go into a Listening state and then into a Learning state, each state lasts the same amount of time as the Forward Delay timer which is 15 seconds by default.
This allows the switch to confirm if opening the port to forwarding will cause loops or not.
Be The Packets - CCIE Study Blog 