Google has long been able to define words for you without sending you to a dictionary website like Dictionary.com, through its knowledge graph. Now, Google has given that card a slight upgrade, and added a search box to that card. That doesn't really sound like a big deal, and almost sounds redundant since there is a big search box above that card. But what this means is that users will be able to search for other words to have them defined without reloading the webpage. This can be very useful when on slower connections, especially those in third world countries where Internet is more of a luxury than a necessity.
This change to Google search has been added in both the desktop and mobile versions of the site, as well as the Google search app on both Android and iOS. You can simply search for a word and add "define" before it to bring up this card. It'll tell you what it means, as well as all of the different meanings it has, and much more. Even telling the user how the word is pronounced. Now, while Google's knowledge graph is pretty extensive, it is possible that you may look up a word that Google doesn't know. And in that instance, Google will still show you search results for that word, from different dictionary websites like Merriam Webster, Oxford and others. So Google is still fairly helpful in that regard.
Google is always changing up its search results to make it easier for its users to find what they are looking for. While some of these changes means that some websites will see less traffic from Google search (since you won't need to click on one of the search results if the answer is in the card), it's still helpful for users looking to find out what a word means pretty quickly. Search is still a huge part of Google's business, with search and Ads together amounting to around 95% of its profit. So it's something that Google, as you'd expect, devotes a ton of resources to, to make sure that it's working well for its users.