"The cakes my mother makes are tasty."
Assuming you mean the 'de' after 'zuo', first, note that the 'that' in the English translation is dropped but is still an invisible reality ("The cakes [that] my mother makes are tasty.".
The de is the link to the direct relative clause (or if you like adj clause) similar to the 'that' in "The cakes that my mother make are tasty." I don't want to give the impression that 'de' is equivalent to 'that' - de is still just a particle, not a complementizer (that) or relative pronoun (who, where etc). It is just used as an attributive with a clause. In English, when an attributive includes a verb, the modifier occurs after the noun as a relative clause by a complementizer (‘that’) so "...cakes that my mother makes...". In Chinese, all attributives precede the noun "makes that cakes."
You can see that direct relative clauses are handled much differently in Chinese and so...
The cakes that my mother makes are tasty = My mother makes that cakes are tasty.
Is it any wonder that Chinese learning English have many problems with direct relative clauses?
YES! And I am forever grateful to the universe for that coincidence. It's also "de" in Spanish. Seems like English is the odd-man-out with "of." Reminds me of pineapples...
I should note that "de" in Spanish corresponding to "of" in English means it doesn't quite correspond to 的 because of the word order. Consider that "of" and apostrophe s are sort of opposites because while they can both show possession you have to swap the words. Usher's house versus the house of Usher.