阳光(yáng guāng) - sunshine, sunlight in Mandarin

阳光 (yáng guāng) is sunshine, the light of the sun itself rather than the star in the sky. 太阳 is the object; 阳光 is what pours out of it and lands on your book, your window sill, your washing line.

It is a plain everyday noun. You sit 在阳光下 (under the sunshine), a room 有阳光 or has none, and a morning can be described simply as 阳光很好. Mandarin also uses it as an adjective for people: 一个很阳光的男孩 is a cheerful, open, uncomplicated lad, and that sense is common in modern speech.

The trap for an English speaker is reaching for 太阳 every time. 太阳很好 sounds like you are complimenting the star; what you mean is 阳光很好. Keep the pair straight: 太阳 rises and sets, 阳光 shines on things.