The "playhead" referred to in a software video or audio app is analogous to the physical device that contacts the magnetic tape of a video or audio tape recorder/player and, depending on the mode (record or playback) either records data to or reads data back from the magnetic tape.
In software, the playhead is usually identified by a line that marks the current location on a timeline of the video or audio data that is being recorded or read back or edited.
To extract a still frame from a video you first have to locate the frame that you want to use. You do that by positioning the playhead at the point in the video where that frame can be found. The still frame you see is usually right where the playhead is positioned.
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