[Yocheved] became pregnant and bore a son. She saw that he was good ("ותרא אותו כי טוב הוא"), and hid him for three months. (2:2)
What does it mean that the baby was "good", and why should this be a condition for its mother being willing to save it?
The best explanation I've seen for this is that "good" means "viable". In ancient times, a large proportion of infants would die early in childhood. If the baby Moshe was likely to die anyway, then it would not be worthwhile for Yocheved to endanger herself in order to protect him. Abandoning your baby to save yourself and your future offspring could not have been easy, but desperate times call for desperate measures.
What is the textual basis for this explanation? For this we can look to another, more famous instance of the phrase "ki tov", in the first chapter of Breishit. After finishing the creation of each part of the world, God looks at it and says that it is good - "וירא אלקים כי טוב".
The Ramban suggests that these comments are not simply a passive commentary on creation. Rather, they are part of the act of creation itself. In the Ramban's philosophy, there is no "clockmaker God" who created everything and then let it run on autopilot except for the occasional miraculous intervention. Rather, the world could not exist for more than an instant, if God did not constantly give it the ability to exist for another instant and another instant, and so on forever.
For the Ramban, "ki tov" in Breishit is the crucial textual indication of this philosophy. When God first created the world, it was only guaranteed to exist for a single moment. But then God called the world "good" - meaning long-lasting, that the world was guaranteed to exist continually, for an indefinite time and perhaps forever. And thus the stable natural processes we are familiar with came into being.
If "good" means "long-lasting" regarding creation, then it might mean the same thing regarding babies. When the baby Moshe was created, he might have been sick and lived only briefly, like the universe which originally would have lasted for just one moment. But regarding both Moshe and the universe, we learn that they were "good" and thus destined to live for much longer.
Even if we do not accept all aspects of the Ramban's cosmology, his understanding of the word "good" may nevertheless be correct and provide a clear explanation of why Yocheved acted the way she did.