Thank you for both of your answers. 감사합니다. ^___^

I agree with billy. There are many(countless) 의태어 in Korean. However, They are usually used for showing giving force to a word, phrases, or sentence, so you don't have to worry about them. you don't have to learning about them seriously.

"꾸벅꾸벅" is 의태어, and shows the action of nodding one's head or upper body continuously.

because we have not grown up in korean society, we underestimate the power of this kind of vocabulary. In my estimate this is the hardest type of vocabulary to master for foreigners. most of these words give a very particular feeling to koreans, something an ordinary, general class of vocabulary cannot even achieve. They usually give a very specific image where the listener can resonate strongly. If you are able to master their usage then I think it's like you have been awarded a black belt in korean studies.