They're a kind of analogue dynamic memory. I'd hesitate to call them RAM because the Access is not Random, but they are a kind of shift register and early computers used those for RAM.
Imagine a pair of MOSFETs connected to a pair of capacitors, and a bunch of those joined together in a chain. All the gates of each one of the pair of MOSFETS are connected together, giving you a "left" and "right" clock input.
When you put a signal in if you pulse the "left" and "right" inputs, it'll store the signal voltage in one capacitor, then pass it off to the next capacitor in turn, like old-timey firefighter handing buckets of water down a line of people.
They used to use this for delaying audio signals before digital memory and analogue to digital conversion was cheap enough to use.
bucket brigades were also used to read large scale sensors like a CCD camera. they are more efficient in their use of die space because you need fewer data paths; they don't need to be digital either, each bucket can be analog for "grey" scale