or; or else
それとも connects two alternative questions, meaning 'or'.
それとも is used when you are presenting two or more options as questions, and you want the listener to choose one. It often appears in casual conversation and is similar to saying 'or' in English when offering alternatives. Unlike または (which is more formal and used in writing), それとも is natural in spoken Japanese.
You can use it with any kind of question: yes/no questions, questions with question words, or even with a noun and ですか. The key is that both parts are questions. For example, you might ask a friend if they want coffee or tea: コーヒーにしますか、それとも紅茶にしますか?
It can also be used after a statement to express a kind of afterthought, like 'or else' as in 'Are you coming, or are you busy?' In that case, the first part is a question, and the second part offers an alternative.