When you don't understand something, you can ask "What?" The word is 什么 (shénme).
You can use it in many questions:

• 这是什么？(Zhè shì shénme?): What is this?

• 那是什么？(Nà shì shénme?): What is that?

• 你叫什么名字？(Nǐ jiào shénme míngzì?): What is your name? (Literally: You are called what name?)