A REC overlay is a camera's on-screen display laid over a video: the red REC light, a running timecode, the battery and the date, framed by corner marks, so the footage looks as if it's being watched through a camcorder. Editors use it for vlogs, found-footage scenes, behind-the-scenes cuts and throwback edits.
This maker draws it in three styles. Camcorder is a clean, modern display. VHS redraws it in a blocky tape-era face with scanlines and a rolling tracking band. Viewfinder shows a cinema camera's thirds grid, center cross and exposure readouts. In all three the timecode starts wherever you set it and moves on one frame with every frame of the video, so it reads like real recording time.
It comes white on pure black. Put it on the track above your footage and set its blend mode to Screen: the black disappears and only the display is left. If you'd rather use chroma key, pick the Green screen look instead.