What is actually happening while a person stands there
It is given as the reason for a rule about spitting.
إِنَّ أَحَدَكُمْ إِذَا قَامَ فِي صَلاَتِهِ، فَإِنَّهُ يُنَاجِي رَبَّهُ
Inna aḥadakum idhā qāma fī ṣalātihi fa-innahu yunājī rabbah
“When one of you stands in his prayer, he is in intimate conversation with his Lord.”
Where this comes from
Munājāh is the word for speaking quietly to someone close enough to hear it. It is not used here as encouragement. It is used as the explanation for why you would not turn your face aside and spit while you are doing it.
The full narration
Narrated Anas bin Malik:The Prophet (ﷺ) saw some sputum in the direction of the Qibla (on the wall of the mosque) and he disliked that and the sign of disgust was apparent from his face. So he got up and scraped it off with his hand and said, "Whenever anyone of you stands for the prayer, he is speaking in private to his Lord or his Lord is between him and his Qibla. So, none of you should spit in the direction of the Qibla but one can spit to the left or under his foot." The Prophet (ﷺ) then took the corner of his sheet and spat in it and folded it and said, "Or you can do this
Source
Ṣaḥīḥ al-Bukhārī · 405
- Agreed upon as authentic (al-Bukhārī / Muslim)