This is the 29th most frequent Chinese word.
还
"还" (hái/huán): 1. [hái] still, yet, also. 2. [huán] to return, give back.
In this sentence, '还' is used to express 'yet' or 'still', indicating that the action has not been completed till the present moment.
我还没吃饭。
I have not eaten yet.
In this case, '还' is used to mean 'fairly' or 'quite', expressing mild intensity of the adjective describing the weather.
今天的天气还不错。
Today's weather is quite good.
Here, '还' means 'to return', describing the act of giving something back to its rightful owner.