A synonym/antonym test is similar to a translation test except that instead of showing you the translation of the item it shows one of its synonyms or its antonyms. Some users may prefer this because the native language is not used in forming questions, therefore no direct translation is done.
Another difference with the translation test is that questions may have more than one correct answer. If the test is setup to use synonyms and an item has more than one synonym then either one is accepted as a correct answer. That is unless you've configured the test to show you the first 1 or 2 letters (or characters) of the answer and only one of the synonyms starts with these letters.