Frozen frames are UI frames that take longer than 700ms to render. This is a problem because your app appears to be stuck and is unresponsive to user input for almost a full second while the frame is rendering. We typically recommend apps to render a frame within 16ms to ensure smooth UI. However, when your app is starting up or transitioning to a different screen, it's normal for the initial frame to take longer than 16ms to draw because your app must inflate views, lay out the screen, and perform the initial draw all from scratch. That's why Android tracks frozen frames separately from slow rendering. No frames in your app should ever take longer than 700ms to render.
To help you improve app quality, Android automatically monitors your app for frozen frames and displays the information in the Android Vitals dashboard. For information on how the data is collected, see Play Console docs.