1, using the charge and discharge of the capacitor, the waveform segment that should have risen or fallen becomes smooth, similar to a straight line, as shown in the following figure, ab; Cd two-segment waveform:

2. In terms of frequency, filtering generally refers to blocking DC through AC, that is, connecting capacitors in parallel in the circuit, filtering out the AC part and letting the DC part pass. In addition, there are high and low frequencies, such as filtering high frequencies through low frequencies. In fact, we often say that DC is also a waveform with very low frequency, so it will be better to know the filtering of high frequency and low frequency!

The rest depends. I hope I can help you. If you have any questions, you can ask.