Throwaway line

When someone says something that is not important or just a quick comment, it is called a throwaway line. Imagine if you were playing catch with a ball and sometimes you throw the ball just a little bit without putting too much effort into it - that's like how someone might say a throwaway line without putting too much effort into it. It's not the main thing they want to say, like throwing the ball really far, it's just a small thing they mention along the way.