Accessibility services are a feature offered on Android devices and is designed to assist users with disabilities or who are temporarily unable to fully interact with their device by allowing an additional and/or alternative interface feedback.
Apps that are not from trusted or official app stores with accessibility service permission enabled on your devices could allow hackers to tap this feature and gain access to your devices.
An app with these permissions enabled will have almost full control over the device. With the right accessibility permissions granted, the hacker can grant him or herself additional permissions and activate your device’s camera and the microphone, capture screenshots, start screen recordings or live stream your screen. They will also be able to install and uninstall any app on your device.