If you don't know what an antonym is, then you already have a disadvantage for the GRE exam. An antonym is an opposite meaning of a specific word. An antonym question will ask the student which word is the most polar opposite of the word listed. This is the time when students need to know their vocabulary. If you don't know the meaning of the word listed, then you have a disadvantage.

Although the GRE is not a memorization test, if you don't have good vocabulary knowledge, then you will have a difficult time with this test. Antonyms are generally easy if you have a good basis for vocabulary. Get some flash cards, or even practice certain words from the dictionary to improve vocabulary knowledge. It will certainly benefit you during the GRE verbal exam.