The broad match modifier is a new keyword targeting technique besides Broad, Phrase, and Exact match types. The broad match modifier allows you to create keywords which have greater reach than phrase match and more control than broad match.
To apply this new match type simply add a plus (+) symbol immediately in front of one or more words. Consider the following diagram:
The above illustration is taken from Google AdWords official blog post which is available at http://adwords.blogspot.com/2010/07/new-keyword-targeting-feature-rolling.html.