What's "crush on" mean here? Is this a commonly used phrase?

Native speaker’s answer
Rebecca
To "crush on" someone is a commonly used phrase! It means to like someone or have feelings for them. It usually used when the other person doesn't know or if they haven't officially told the other person that they like them. Ex: I have a huge crush on this girl in my class. Should I ask her out? Ex: Jen has a crush on Jim. But Jim doesn't know that.