What is salat-ul-safr?

Answer 1

salatul-safar

Shortening prayers. The meaning of shortening prayers is that the four-rak'ah prayers become two rak'ahs when travelling.

As for joining prayers, it means that the worshipper joins two prayers, Zuhr and 'Asr, or Maghrib and 'Isha', at the time of the earlier or later of the two prayers.