'To bubble in' is a phrasal verb. In this context, it means to fill the small circle next to a multiple choice question on an exam, to make the circle black to show what answer you choose. The small circle on an exam is called a bubble, so that's why we use 'bubble in' as a verb here. 'To bubble' is a regular (non phrasal) verb in other contexts, meaning it makes bubbles - like a cup bubbling with soda. I hope this helps!