An antonym is a word that has the opposite meaning to another word.
Many antonyms are formed by adding the prefix un- to an existing word.
The prefixes mis-, dis-, in-, ir- and im- and il- are used instead of un- with some words to create antonyms.
Mis- usually has the meaning of doing something wrongly, such as misinforming someone about something.
However, most of the prefixes mean exactly the same thing as each other, so it is just a case of learning which ones should be added to particular words.
You may find a dictionary helpful for this activity.