Google announced that they are going to be boosting the effects of the mobile-friendly algorithm, Mobilegeddon, which they launched in April of 2015. This gradual rollout will start to happen this May and it will increase the effect of the ranking signal in mobile searches. Since the algorithm is a page by page signal, it could take time for Google to crawl through all of the pages on a site. This is why when they announced the start in May, that it will be a gradual boost and the impacts could be slow because of it.
The last update, Mobilegeddon, impacted mobile searches in a way that mobile friendly site would appear more often than non-mobile sites. This new update is said to not have much as an impact as Mobilegeddon, but boosts websites nonetheless. Make sure that your websites and pages are mobile-friendly so you can have an advantage with mobile searches.