Waterfall diagrams like this are used in amateur radio to show activity in a frequency range. They are now integrated in many modern devices and are also used in programs for digital operating modes and for SDRs. This waterfall diagram shows 50 parallel FT8 transmissions of 50 Hz width each in a small frequency range of approx. 2.5 kHz. The duration of a transmission (vertical Acse9 is 15 seconds and is synchronized with a time standard (e.g. GPS). The decoding takes place, for example : with the program WSJT by Joe Taylor ( amateur radio call sign K1JT ).
FT8 is a digital operating mode used in amateur radio for wireless communication. It is mainly used by radio amateurs on shortwave, but in principle also on all other frequencies. Radio amateurs have developed the WSJT program for this purpose, with which one can work in this operating mode. The screen excerpt shows a few lines of the WSJT source code.
FT8 was publicly proposed in 2017 by Joe Taylor ( amateur radio call sign K1JT ), who is well known not only among radio amateurs and quickly grew in popularity.
It is characterized by the rapid transmission of data within a time frame of fifteen seconds (15 s) with a simultaneously low required signal-to-noise ratio (SNR). The modulation used achieves a transmission with a very low bandwidth of 50Hz (for comparison: telephony / speech requires at least 2.7kHz, videos from 100kHz depending on the quality). It can be used to cover very large distances with little power and simple antennas.
Link: What is amateur radio?
Link: What is USKA?