It records what you share with the browser's screen-sharing dialog (a single tab, one window or a whole screen) into a video file, using the browser's own getDisplayMedia and MediaRecorder. You can add the sound of the tab or of the system, add your own microphone for a voice-over, pause and resume, and download the result. The video is built in your browser's memory and saved straight to your disk; nothing is uploaded, and there is no watermark or account.
What is and is not captured
The browser decides what can be shared, not the page. A tab recording follows that tab only, and in Chrome and Edge it can include the tab's sound. A window recording shows that window, including whatever covers it on some systems. A screen recording shows everything, including notifications that pop up, so turn on "do not disturb" first. Whole-screen system sound is offered by Chromium browsers on Windows and ChromeOS; on macOS it depends on the browser version and the system, and Firefox and Safari record no shared sound at all. When that happens, the panel says "not recorded" for tab or system audio, and the microphone is the only sound you can add.
The mouse pointer is normally included. The page you record with is excluded from the list on purpose, to avoid the "hall of mirrors" effect of recording the recorder.
Chrome, Edge and Firefox produce a WebM video (VP9 or VP8 with Opus sound); Safari produces MP4 (H.264). The file name matches what was produced. WebM plays in every modern browser and in VLC; if an editor refuses it, convert it there or with a desktop tool. Recordings stop by themselves after 60 minutes, because the whole video is held in memory until you save it. Phones and tablets do not offer screen capture to web pages at all, so on mobile use the recorder built into the system.
To blur names or numbers in a still frame before you share it, use the blur and redact tool. For voice-only takes, the voice recorder gives you a level meter and WAV export. The guide recording your screen in the browser covers the choices in more depth.