Every word has a synonym (same meaning) and usually an antonym (opposite meaning). The 11+ exam asks you to find these at speed, often using words you have not seen before.
Think of it as direction: synonyms point the same way, antonyms point the opposite way.
A synonym means the same; an antonym means the opposite. On the 11+ exam, you will need to find the closest match or the direct opposite for words you may not have seen before.