What does downloads mean in Google Play store app page?

It's the total over the life time of an app.
In 2017, Google Play Services was the first app to reach 5 billion downloads.
The important point from the article is:

If you're not familiar with how the Play Store works, it's important to note that Play Services hasn't actually been manually downloaded that many times. When someone logs into an Android device with their Google account, Google Play marks the pre-installed apps as downloads. So in reality, this means over five billion devices with Play Services installed were used at some point.

Whether an app was downloaded directly via Google Play or whether it was installed by a manufacturer, Google Play Store counts both occasions as downloads and adds them to the total.

With Google's announcement that there are 2.5 billions (excluding China and Amazon's FireOS) active devices running Android, the number of downloads shown on the app's Google Play page can be skewed if it comes already installed on the devices.