Synonymy in Japanese means that there are different words that have similar meanings. This means that even though they may be different words, they can mean almost the same thing. For example, the words "big" and "large" are synonyms in English. Similarly, there are many words in Japanese that have similar meanings.
So, if someone is speaking Japanese and they use one word that has a similar meaning to another word, then it won't really matter which word they choose because the overall meaning will still be the same. It's just like when someone is drawing a picture and they use a green marker or a blue marker - it doesn't really matter which color they use because the picture will still look the same.
However, there are times when one particular word may be more appropriate to use than another word. This depends on the context or situation. For example, if someone is talking about the size of a building, they may use the word "大きい" (ookii) which means big or large. But if they are talking about the size of a person, they may use the word "背が高い" (segatakai) which means tall.
So, while synonymy means that there are different words that have similar meanings, it's important to choose the right word for the situation.