Learn French: Celles-ci ont l'air très maniables en ville. - These ones look very maneuverable in the city.

The expression avoir l'air translates to "to seem" or "to look". When the subject of the sentence is an inanimate object, the adjective following avoir l'air must agree with that subject, not with the word air. Because Celles-ci is plural, maniables takes an -s.

Would the agreement rule be different if Celles-ci referred to people instead of things?

For people, the adjective usually still agrees with the subject (for example, Elles ont l'air heureuses). However, with people, it is occasionally acceptable to make the adjective agree with the masculine singular noun air if you are strictly describing their facial expression, but agreeing with the subject is always the safest bet. With objects, you have no choice: it must agree with the subject.

What does the -ci add to the pronoun Celles here?

Celles by itself means "the ones" (feminine plural). Adding -ci makes it Celles-ci, meaning "these ones" (indicating things closer to the speaker). It is often used to contrast with celles-là, which means "those ones".

Why do we say en ville instead of dans la ville?