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An oscilloscope plots a graph. In order to have it visible, the graph must be refreshed regularly.
The dot moves across the screen at a rate determined by the sweep circuit; in old units it was calibrated in sweeps per second but modern units are calibrated in time per division. It's controlled by the sweep circuit controls.
To get a steady pattern you need to synchronize the sweep with the signal. You can use the signal itself as a synchronizing source, an external signal, or the power line frequency.
This particular unit also has a detector for RF, called a demodulator. That extends its usefulness a bit, but there is no substitute for looking at the signal itself rather than just its modulation.
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