On Windows 10, "Controlled folder access" is an intrusion-prevention feature available with Microsoft Defender Exploit Guard, which is part of the Microsoft Defender Antivirus. It's been designed primarily to stop ransomware from encrypting and taking your data hostage, but it also protects files from unwanted changes from other malicious applications.
The anti-ransomware feature is optional on Windows 10. When enabled, it uses a mechanism to track the apps (executable files, scripts, and DLLs), trying to make changes to files in the protected folders. If the app is malicious or not recognized, the feature will block the attempt in real time, and you'll receive a notification of the suspicious activity.
