The literal meaning or original intention of nasal bubbles is that nasal mucus is blown into bubbles by the airflow in the nose. In the picture, the nose bubble hangs on the child's nose. But in ordinary conversation, it may have extended meaning. For example, someone says to you, "Is this an eye bubble or a nose bubble?" The nose bubble here means that you didn't see what you should have seen, which means a little blame and banter.