“一颗心”是“一”加上量词“颗”，再加上“心”，用来说一个心。

"一颗心" is composed of "一" plus the measure word "颗" and "心", used to refer to one heart.

人们常用它来说自己的感情，比如“一颗真心”。

People often use it to express their feelings, for example, "一颗真心" (a sincere heart).