Fig.1 shows how the current through diode changes when voltage accross the diode changes. When forward bias is applied across diode then a large number of electrons from the n typee material progressing through the p-type material and large number of hole in the n-type material. As a result forward current starts flowing.

As applied voltage is reversed, a reverse bias situation is created and because of large number of minority carriers in each material the diode current will simply reverse as shown in Fig 2. and stay at this measurable level for period of time ts(storage time) required for the minority carriers to return to their majority carrier state in the opposite material. Eventually, when this  storage phase is passed the current will be reduced in level to that associated with the conduction state. This period of time is denoted by  t* (transition intervel). The reverse recovery time is the sum of these two intervels

$\therefore t_{rr}=t_s+t_t$