What is the reverse recovery time in a diode?
If a diode is conducting in a forward condition and immediately switched to a reverse condition, the diode will conduct in a reverse condition for a short time as the forward voltage bleeds off.  The current through the diode will be fairly large in a reverse direction during this small recovery time.

After the carriers have been flushed and the diode is acting as a normal blocking device in the reversed condition, the current flow should drop to leakage levels.

This is just a generic description reverse recovery time.  It can affect quite a few things, depending on context, as mentioned in the comments.