Power Platform is a low-code platform empowering citizen developers as well as professional developers enabling e.g. apps to be created and automation to be set up. The four main services are: Power BI, Power Apps, Power Virtual Agents and Power Automate. AI Builder, Common Data Service and Connectors are also parts of Power Platform.
Once up on a time there was a CRM-system from Microsoft called Microsoft Dynamics CRM. Actually the story started even earlier when Microsoft bought a CRM company consisting of 10 employees in 2001. Two years later Microsoft CRM was born. It was later rebranded to Microsoft Dynamics CRM. This was more than a CRM-system. Already back then it was called an "xRM" platform rather than just a CRM system. You could create your entity models and solutions which could support and let the users manage any (X) relationship rather than just Customers (C). Some say it also referred to xRM as in extended relationship management.
Parts of (or at least concepts from) the Microsoft Dynamics CRM/xRM platform has been lifted into Power Platform (CDS and model-driven apps as). Then new cloud concepts have been added as well making up other parts of Power Platform (such as e.g. canvas apps and Power Automate).
In 2019 Microsoft was recognized by Gartner as a leader for Enterprise Low-Code Application Platforms Magic Quadrant. Gartner names Microsoft a Leader in Enterprise Low-Code Application Platforms 2019 Magic Quadrant
E-book - Migration to the cloud by Alex Shlega
How to make it from model-driven apps to canvas apps blog post by Carina Claesson, includes a chapter "From xRM to Power Platform"