Some time ago, Google announced that they would be updating third party app connections in order to make them more secure. Most developers have moved their apps to the new authentication protocols by now, but some have not, which will result in messages similar to the one below.
The message does not mean your account has been accessed outside of your control, nor does it mean someone is trying to gain access to your account.
Unfortunately, we do not have any way of knowing which app is generating the message. In some cases, it could be a web app. Other times, it could be an out-of-date mobile app or operating system (Kindle's have been known to cause those messages). However, you can check to see which apps you have granted access to your Google account by visiting this page:
https://myaccount.google.com/permissions
If you are no longer using an app, you may remove it by clicking on the app and then clicking the "Remove" button. Removing an app does not guarantee you will stop receiving the blocked messages, but it can help narrow down possible culprits.
You can also check device activity on this page:
https://myaccount.google.com/device-activity
For all devices listed that you still use, make sure you have updated them to the latest operating system and app versions.