This is the 248th most frequent Korean word.
해
"해" in Korean means "sun" or "year," depending on context.
In this sentence, '해' means the sun.
해가 밝게 빛나고 있다.
The sun is shining brightly.
Here, '해' refers to a year, as a time period.
내년에 해낼 목표를 정했어요.
I set goals to achieve next year.
In this context, '해' means to do something, stemming from the verb '하다' (to do).