A level is a small part of N. There are 500 of these things, and each one is more difficult than the last. Each level has a name, as well as a number (such as 00-0: 'The Motherlode'). There are 5 levels in an episode, and 50 in a column.
To get a more exact definition of a level and to see why maps and levels are different things, visit the maps portal. In the menu. Some levels contain a demo of failing it. Here are some of them 03-4 99-0 11-4 59-1 82-3 98-3 05-1 etc.
If you would like to create an article about a level, go to the episodes portal, select the corresponding episode. On the next page, choose the level's link. On the next page, insert {{subst:Level}} and save the page once. A template will be created informing others that you are creating the article at the moment and giving you some advice about how to create the article.