Synonyms are words that mean nearly or exactly the same as another word. If you're looking for a list of them then I would recommend the website English Leap.
English Leap is a website that lists a broad range of synonyms that are listed in alphabetical order.
This website doesn’t list meanings, but Smart Words does. To find the meaning of the words, you simply have to hover your mouse over the red, emboldened word and a pop up box will appear with the meaning.
This website lists a broad range of synonyms for each and every word, so it is probably a lot more helpful.
Synonyms are used to make a sentence richer and more colourful. They really come in handy when you are searching for a word and just can’t find it – a quick search for a synonym will often give you the answer you are looking for.