Learn Turkish: Onun elbisesi kısa.

In Turkish, when something belongs to a third person (his/her/its), the object being owned gets a possessive suffix. Since elbise (dress) ends in a vowel, we use the buffer letter s before adding the 4-way harmony vowel i, making it elbisesi.

If onun already means 'her', why do we also need a suffix on the dress?

Turkish uses double-marking for possession. You use the possessive pronoun onun (her) AND you must attach the possessive suffix -si to the object owned. Even if you dropped onun from the sentence, you would still need to say elbisesi to show it belongs to someone.

Does onun elbisesi only mean 'her dress'?

Turkish pronouns do not indicate gender. Onun can mean his, her, or its. We translate it as 'her' here just as an example, but without extra context, it could just as easily mean 'his dress'.

Where is the word for 'is' in this sentence?

There isn't one! In Turkish, you do not need an ending for 'is' when making simple third-person statements like 'the dress is short'. Simply putting the subject onun elbisesi and the adjective kısa together does the job.