A roadblock ad refers to when one advertiser gets all of the available ad units on a page.
Often roadblock ads consist of different types of ad units, such as banner ads and text ads. With a roadblock ad, a single advertiser is given all available ad units, from banner ads, to text ads, to sidebars.
Roadblock advertisements are effective in driving engagement for two reasons: One, using a mix of ad unit types increases the probability the ad is successfully delivered, and two, the advertiser gains 100% share of voice on a given page.