It's a common phrase used to express that the day is going well, or to acknowledge that something is good or positive.
Here's a breakdown of the characters:
* 今天 (jīn tiān): Today
* 很 (hěn): Very
* 好 (hǎo): Good
It's important to note that "今天很好" is a general statement. It doesn't necessarily mean everything is perfect, but rather that the day is going well overall.