Sunnah Mo'akkida: as Athari said, are prayers the Prophet never left but are not wajib.
IF The fard salah is offered, the obligation is fulfilled however if there was no having-less-time problem, it is recommended to offer sunnah as well..
The difference between wajib and sunnah mo'akkida is that we are bound to offer a qadha prayer of any leftover wajib as well [in case of wit'r NOT Eidain] but once the time is up there is no qadha of sunnah and no gunnah of not doing it... THere is a hadith according to which at the day of judegement when the prayers are accounted, if after fard prayers are being take care of any nafil and sunnah prayers will be extremely helpful that day...
meree jaan, sunnah toh khuda ka pyaar heh, woh pyaar jis neh humareh naby koh ishq meh fanaa kur dee ya tah, uggur sunnah pur nee heh yeh sochna, Humareh hazoor neh puree teeh, unka mehkaam itna uncha heh, and becuase of my love for the prophet saw and for Allah t'aalah im also gonna read them.