What are those things spraying something at the pad during the beginning of a Falcon 9 launch?
It is a sound suppression system like the one developed by NASA for the Space Shuttle in the 1980s. It was required to keep the sound level below the required 145 dB. See: https://www.nasa.gov/mission_pages/shuttle/launch/sound-suppression-system.html
On launch pad 39A in Cape Canaveral, a water tower dumps 300,000 gallons of water in less than a minute during take off. I am not certain of the specs (gallons and noise levels) on this particular launchpad.
Water-deluge systems have long been a common part of large rocket launches. It supplies sound suppression for the launch and also prevents the back-blast from the engines from critically damaging the launch pad or rebounding from the pad back into the rocket itself.