1kHz Laser Detection
They use a solar cell as a photo-detector and it looks like they modulate the laser with a square wave at 1kHz.

After the solar cell, the signal connects through a bandpass filter to "reject" dc (from sunlight) and all frequencies other than those close to 1kHz. The square wave now looks like a sinewave (because of the filtering) and this can be seen on the oscilloscope.

They seem to imply (in the written words below the video) that they have "tightened" the bandwidth of the low pass filter using a phase locked loop (PLL). I suspect they are using a switch-capacitor filter at this point because of the mention of the PLL.

This improvement will give a much tighter pass-band and once "something" is detected, the PLL will produce a multiple of the 1kHz (maybe 64kHz) to clock the switched capacitor filter.

Another advantage of using a PLL is that if the originating frequency drifts slightly, the PLL can be set to track it. So, if it drops down to 990Hz, the PLL will produce 63.36kHz and keep the switching filter "in tune".

The PLL might also set limits for upper and lower "acceptable" modulation frequencies. This is another advantage using this method.

A great switched capacitor filter is this part, the LTC1068 as recommended to me by Anindo Ghosh.