Having looked at many different flight controllers I saw that many of them have filtered power pads. What does it mean when it says filtered power?
Without knowing more specifics about the board(s) you're asking about, it's more than likely that the filtered power pads on your flight controllers are just the normal output from the 5v voltage regulator on the board which has a capacitor between the voltage regulator output and ground.
Because the 5v regulator's output is shared by all kinds of devices connected to the board, all of which consume different amounts of power and at different times, it's entirely likely that the actual voltage on the regulator's output will have some "ripple" where the voltage oscillates around 5v.
Some components on the drone which deal with analog signals, like the camera and VTX, may introduce visual artifacts into the image or intermittently drop out if they are fed power with too much ripple. The aim of the filtered voltage output is to smooth out the ripple so the voltage oscillations are far less pronounced. In the above graph, the solid black line is the filtered output.