It means "from one point of view, the answer is YES, from another point of view, the answer is NO" you use it when you want to show that an answer is more complicated than it seems

Cristophe is correct. Here's an example: Q: Do you think humans are responsible for the climate change? A: Yes and no. There has always been a change in the climate of the earth throughout history, but on the other hand it seems that it's going quite fast these days. So perhaps we are responsible for the rapid change in today's climate.

或许吧？ A more question: can we use humans instead of human?