what is the point of tiny ceramic caps on power rails

[nus]

so i just replaced the c15 100uf cap on 3j1-2 and there is 4v of ripple on the power rail. it was all over the place before this.
probably going to replace the varistor and the ceramic cap on the line now, however i was wondering what is the point of these tiny caps, they always seem to be present on main power rails even after a larger cap

[ataradov]

Here is a video on it

[Richard Crowley]

https://youtu.be/BcJ6UdDx1vg

https://youtu.be/P8MpZGjwgR0

https://youtu.be/1xicZF9glH0

[bson]

It's there to reduce high frequency ripple because the electrolytic cap's impedance climbs quickly with frequency so it can't do it alone.

It's not technically a decoupling cap - those go across loads to reduce high frequency loop currents.