An antonym is a word which means the opposite of another word. Antonyms are also called opposites. One way of mastering a language involves learning its vocabulary. Synonyms and opposites are helpful in this sense
Two opposites of "light" are "dark" and "heavy".
Here is a list of opposites arranged in alphabetical order.
NOTE: this list of opposites is of course by no means an extensive one. More opposites can be added.