Hi Zhao! "Obese" means that the person carries so much weight that their health and lifespan are affected. If they don't do something to lose the weight, the person will have many health complications that lead to a shorter lifespan. "Chubby" means that the person may carry a little extra weight but it won't affect their daily lives. The person will still be able to do normal physical activities without concern for personal health. "Chubbiness" is not considered life-threatening. Another similar word is "pudgy". "Fat" can be used to describe a person who is "chubby", or "obese". It is used in a more general way. I hope this helps!

They are all synonyms but there are some important differences between these words. Obese is a more medical/clinical word to describe someone who is overweight or fat. Chubby is a nicer and more euphemistic word for fat.

They all mean 'not thin'. Fat is a general term for someone who is not thin. Many fat people prefer the term fat to the others, because obese implies a negative judgment (along with overweight, see: http://thisisthinprivilege.org/faq#overweight). Chubby is a word used to avoid the negative value of fat but can still be a pejorative or a sexual fetish-ization term. So fat activism just uses 'fat'. Although, when talking to someone who is fat, you will probably not need to describe their body to them unless you are going to be rude. See: http://english.stackexchange.com/questions/177244/a-polite-way-to-say-that-someone-is-fat