Ṣalāt al-Duḥā is a voluntary prayer offered in the forenoon — a beloved habit of the Prophet ﷺ.
When: from about 15–20 minutes after sunrise until just before the sun reaches its peak (before Dhuhr). The best time is when the heat of the morning rises.
How many: a minimum of 2 rakʿahs, and 4, 6 or 8 are all authentic — praying two by two.
Why: it is a daily charity on behalf of every joint in your body:
"Every morning charity is due on every joint... and two rakʿahs of Duḥā suffice for all of that." — Ṣaḥīḥ Muslim
The Prophet ﷺ advised his companion Abū Hurayrah to never leave three things, among them "the two rakʿahs of Duḥā." A small prayer, a lasting light in your day.