The store review action triggers a review request for the relevant application store for the user's device, Play Store for Android devices or App store for iOS devices. 

How to add the store review action

Prompt app store rating is an additional action that's available with the Dismiss Flow action. 
To add this action, select a button in a flow, change the Button action to Dismiss Flow, and toggle on the Prompt app store rating option. 

When a user presses the button the flow will be dismissed and an attempt will be made to trigger the Play Store or the App store rating depending on the device.

When to use a rating request

Rating requests are typically used as part of a user feedback journey. 
With this setup, a user would be presented with an experience asking users how they're enjoying the application. Based on their response users with a positive experience would have the app store rating triggered, users with a negative experience would have a feedback step triggered.

When setting up a flow that uses the rating request the frequency should so users will see it with the same frequency that a rating can be triggered for their device and other flows that can trigger the rating request should not be shown in this time frame.

How Google and Apple rating requests work

Google provides general guidelines about Play Store reviews in their comment posting document. Play Store reviews should not be triggered more frequently than the quota that Google has set.

Apple provides their guidelines for App Store ratings in their ratings and reviews document. They limit ratings to three occurrences in a 365 day period.

Android devices that don't use the Play Store

Some Android devices don't include the Play Store. In these cases the review action can be excluded. Reference information is available in the developer docs here.