What's "better be" mean and when's it used?

Native speaker’s answer
Rebecca
"Better be" means what someone "should be" doing or experiencing. It implies a kind of threat or consequence if that thing is not done. But it can be used seriously or casually, usually in conversation. Ex: You'd better be doing your homework. Otherwise, you're grounded this weekend. Ex: She better be happy we're going to the movies this weekend since I'm missing my meeting for it.