A blog carnival is a post in a blog that summarizes a collection of articles from many different blogs on a specific topic. The idea is to collect some of the best content on a topic in a given week. Often many other blogs link back to a carnival host and as such the people that have articles featured in the carnival often enjoy a spike in new readers.
This concept can be confusing so I suggest you take a look at the Carnival of the Cats for an example. You can also find a list of all the carnivals and submit your articles at the Carnival Submit Form.