It works similarly to pre-roll ads for other videos you scroll past in your timeline. When you get to a Periscope clip, that footage can begin with a short ad. Twitter calls the new push "a new source of brand-safe video advertising" that companies can use to promote products, services and more. It's also a way for creators and publishers to monetize the content they're streaming on Periscope all the time.
The pre-roll Periscope ads are available to select advertisers now, but they will "roll out more broadly" in the months to come. You might not notice them immediately, but eventually the new brand clips will likely show up ahead of live video or archived replays in your Twitter feed. Today's news comes on the heels of the announcement that livestreamers could use Periscope with third party apps and devices last week. Facebook continues to make live video a key focus and it looks like Twitter is expanding its streaming footprint as well.