A synonym is a word that means the same thing as another word. If you replace a word in a sentence with its synonym, the meaning of the sentence won't really change that much.
There are, of course, shades of meaning, and not every synonym is an exact substitute, but sometimes we just need a different word for a change. Some words have dozens of synonyms, such as funny: amusing, hilarious, laughable, slapstick, witty, and many others. Other words, sadly, have no synonyms, like postmodernism. Check your thesaurus (dictionary of synonyms) — it's all there!