Roadblocking is serving several creatives from a line item together on the same webpage. It's called a roadblock because it blocks off the ad slots on a page and fills them with creatives from one line item.
Roadblocks are set up within line items. They can be set up to work with all of the creatives in the line item, or to work with master/companion creative sets, where a master creative is always delivered first.
Guaranteed and non-guaranteed roadblocks
Guaranteed roadblocks are roadblocks where DFP guarantees that multiple creatives will be served together on a page. Guaranteed roadblocks only work with Google Publisher Tags in single-request mode. With these tags, all of the creatives for a webpage are requested at once, so DFP can select groups of creatives and serve them together.
Non-guaranteed roadblocks work with any tagging method and serve as many creatives together as possible from a single line item, but there is no guarantee that all creatives will be served.
Roadblocks and frequency capping
If you've set a frequency cap for a line item, DFP considers each delivery of the line item as a single view toward the frequency cap, even if multiple creatives are delivered together. This is true for all roadblock types, even if multiple impressions are counted for reporting purposes.