Dubbed “Mobilegeddon,” Google’s latest tweak to its search algorithm arrived on Tuesday. It could make a significant impact on how company’s websites are ranked in search results on smartphones.

The adjustment will now favors sites in search that are more mobile friendly. That means companies and retailers that have sites that meet Google’s mobile standards will rank higher in search results on phones.

As the search giant announced in February, the change “will affect mobile searches in all languages worldwide and will have a significant impact in our search results. Consequently, users will find it easier to get relevant, high quality search results that are optimized for their devices.”

According to marketing agency Merkle/RKG, as of early April, 46% of Fortune 500 companies and 29% of the top 500 retail sites had not received Google’s “mobile-friendly” designation.

Mobilegeddon could mean a drop in traffic for these websites that don’t meet Google’s standards for being smartphone friendly. Google GOOG even launched an online an online test for companies to determine whether their site meets the search engine’s criteria.

What constitutes a mobile friendly site? Google says that websites need to have a layout and text that is large enough so people don’t have to zoom, or scroll horizontally. Users should also be able to easily click on links via a touch interface.

As more people search on mobile and access the web via smartphones, Google has to make sure that it presents the highest quality search results to users. Retailers, specifically, will need to make sure that their sites meet Google’s standards.

It’s important to note that this will not take effect on tablets or desktops. And Google says that this is a page-level change, which means that it will only effect the pages of your site that are not mobile friendly.

We won’t be able to know who algorithm affects for another week, the company says, as Mobilegeddon won’t fully expand to all pages indexed until next week.

Even so, Google isn’t ringing the alarm bells to loudly. As the company writes, it still uses a “variety of signals to rank search results,” and a page with high quality content could still rank high even if it is not mobile friendly.