A minor change on Simple Wikipedia means that the changes you make to an article are very small (minor means "small") and no one would argue with them. For example, fixing grammar or spelling would be a small change. Any change to an article that changes its meaning is not a small change. Any change to the information, pictures, or other things in the article is not small, and should not be marked as small. For example, adding simple formatting, fixing wikilinks, and removing vandalism are small changes. Writing a comment on a talk page is not a small change.
To mark something as a small change, check the box beside the text saying "This is a minor change". This will help other editors know that it is not a big change if they are looking for something.