The Power Apps Pay-as-you-go plan is one of the plans available for purchase within Power Apps licensing (Preview 2021-11-29).
Link an Azure subscription to an environment. The users then use Power Apps and/or other “meters” (Dataverse database capacity or Power Platform requests). Bills can be views, costs can be analyzed in Azure Portal. You pay for monthly active users and an active user is a user who opened an app or a portal at least once a certain month. 1 GB database and 1 GB file capacity is included. Scale beyond and pay for what you use ($48 extra per GB, make sure to design your apps wisely!).
The plans available are now Per App Plan (pay per user/app/month), Per User Plan (pay per user/month) and NEW! Pay-as-you-go Plan (pay per ACTIVE user/app/month).
Read more about Licensing
At Microsoft Ignite November 2021 a new Power Apps licensing alternative was announced, Pay-as-you-go plan.