What is meant by the black and white threads in the verse on fasting

What is meant by the black thread in the verse is the night, and the white thread
is the dawn, not the rising of the sun. 

Dawn is called the white thread because the first thing that is seen of the dawn
is a light in the sky that looks like a thread along the horizon, stretching from right to left, from north to south. Then it continues to
increase until it spreads through the whole sky. See Fath al-Baari, commentary on hadeeth no. 1917. 

Al-Bukhaari (1916) and Muslim (1090) narrated that ‘Adiyy ibn Haatim (may Allaah
be pleased with him) said: “When the words ‘until the white thread (light) of dawn appears to you distinct from the black thread (darkness of
night)’ [al-Baqarah 2:187 – interpretation of the meaning] were revealed, I took a black rope and a white rope and put them beneath my pillow,
and I started to look at them during the night, but I could not see any distinction between them. The next morning I went to the Messenger of
Allaah (peace and blessings of Allaah be upon him) and told him about that. He said, ‘Rather that is the blackness of the night and the
whiteness of the day.’” 

Al-Nawawi (may Allaah be pleased with him) said: 

Abu ‘Ubayd said: The white thread is the true dawn and the black thread is the
night. 

And Allaah knows best.