The idiom être à (to belong to) is usually used to show possession of a thing. Conjugate être to agree with the subject, and if the sentence contains two or more nouns, repeat the preposition à before each noun: Ces CDs sont à Mathieu et à Bernard. (These CDs belong to Mathew and Bernard.)
Note how être à is used in questions, as follows:
- À qui est ce stylo? (Whose pen is this?)
- À qui sont ces magazines? (Whose magazines are these?)