Google said in February that it was going to make mobile-friendliness matter more to its search-engine rankings. On Tuesday, it makes good on that commitment -- and webmasters who didn't heed the warning may see a steep drop in traffic.

It's a shift so potentially fraught with peril for those who aren't ready that it's being called "Mobilegeddon." Essentially, Google is expanding its use of mobile-friendliness as a ranking signal when it compiles search results. Sites that are mobile-friendly will be ranked higher in search results; those that aren't will suffer.

[See also: How to prep, in 7 steps, for Google's mobile search change.]

The change will affect mobile searches in all languages worldwide and will have "a significant impact in our search results," the company explained in a post announcing the change earlier this year on its Webmaster Central blog.

The idea is to improve search on mobile devices, and Google has provided tools to help webmasters ensure their sites live up to the new requirements. In addition to its guide to mobile-friendly sites, the company also offers a testing tool to assess mobile-friendliness along with full mobile usability reporting.

Sections of sites owned by Wikipedia, the BBC and the European Union all failed the Mobile-Friendly Test, the BBC reported on Monday.

"There's significant evidence that many large sites are still not mobile-friendly," said Greg Sterling, vice president for strategy and insights with the Local Search Association. "Google is trying to get mobile laggards to address the smartphone user experience."

By rewarding sites that are mobile-friendly, Google helps improve its overall mobile-search user experience, which in turn encourages more mobile-search usage, Sterling explained.

"If consumers have negative or frustrating experiences with mobile search, they'll be less inclined to use it," he said. "Google is more vulnerable in mobile than on the PC. It must continue to improve the mobile-search experience and make it competitive with apps."

It's possible some websites will experience a decline in traffic once Tuesday's changes take place, Sterling added, requiring a redesign of the site or at least the affected pages for better usability on mobile devices.