What is the meaning of doing a voluntary Salaah (prayer) of two Rak‘ahs?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

This means to perform a prayer of two Rak‘ahs other than the five obligatory prayers, whether during the day or the night.

This is mentioned in the prophetic narration on Istikhaarah that was narrated on the authority of Jaabir,. Allaah Knows best.