On the page edit form, the "minor edit" checkbox allows you to indicate that the changes you are making are just minor (such as correcting spelling mistakes, changing keywords, making the page look right...) and are thus not worth logging to RecentChanges.
This helps to keep RecentChanges tidy.
If you only change the way a page looks, it is a minor edit. If you add, change or remove content, it isn't.
To ignore this distinction and view all recent changes, major or minor, visit FullRecentChanges.
Also see consecutive edits.