Learn Turkish: Onun sayesinde çeyiz için neler hazır? - Thanks to her, what all is ready for the trousseau?

The word neler is the plural form of ne (what). You use it when you expect the answer to be a list of multiple items, which translates well to 'what all' or 'what things' in English. Because preparing for a wedding involves many different goods, the speaker naturally assumes there are multiple things ready and asks neler.

Does the word sayesinde always need the genitive case like in onun sayesinde?

Yes, sayesinde (thanks to) grammatically functions as a possessive noun compound, so the person or thing you are crediting must take the genitive case. This is why we use onun instead of just o. As a quick refresher: remember to only use sayesinde for positive outcomes! If you want to say 'because of her' with a negative or blaming tone, you would use onun yüzünden instead.

What exactly is a çeyiz in Turkish culture?

A çeyiz is a traditional 'trousseau' or 'hope chest'. It is a collection of household goods, linens, lace, and other decorative or practical items that a young woman and her family prepare and save up for her future marriage.

Since neler is plural, why isn't the predicate pluralized as hazırlar?