The Topic Particle は
The particle は presents the topic of one's utterance ("As for item X, it is such that..."). It puts forward the item that you want to talk about and comment on. A topic phrase, however, need not be the subject of a sentence.
Ex: メアリーさん、しゅまつはたいていなにをしますか。 (Mary, what do you usually do on the weekends?)