Google Maps is currently pushing new notifications to some users on the web, as opposed to just through the Google Maps app. It appears to only be for those that are Local Guides, at this point but that will likely change in the near future. These notifications are currently alerts which are "reminders to review and share photos for places you've visited". This is going to make it easier for Local Guides to send in pictures of a particular place, as well as leave reviews. Which is what the Local Guides program is all about, users posting photos, writing reviews and even providing details about a location.
The Local Guides program has been a big part of Google Maps for quite some time. In fact, Google Maps even offers some benefits for those that are Local Guides. Users will get early access to products, some free Google Drive storage and even access to some exclusive events. It's not a well-known program, but it does allow Google to help keep Maps up-to-date as well as provide even more information about each location that is on Google Maps.
Users that do have the notifications available - which is a server-side switch, and again it's only available for Local Guides members right now - will get a pop up to allow notifications from Google Maps. After you agree to allow notifications, you'll now see a new notifications menu control in the navigation drawer, that slides out from the left-hand side. So you'll be able to control which notifications you receive from Google Maps in the future.
Google Maps is following YouTube's lead in bringing notifications to the web. Not too long ago, YouTube started notifying users on the web when their subscriptions posted a new video. So that users could simply click on them and watch that new video. The idea with Google Maps is basically the same. Allowing users to write a review at that restaurant that they had lunch at, especially if they were in a rush and seemingly forgot to do so while they were out and about. There's no word on when these notifications will be available for other users just yet, but it's likely in a testing phase at this point.