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i think you talk about bypass capacitor - it's role to separate dc bias voltages. For example, dc voltage at port (it is grounded) is thus 0V, and dc bias voltage at input of amplifier, for example, equal to base-emitter voltage of bipolar transistor, so you need to isolate these points at dc, but couple at high frequencies - so we use large capacitor, which has infinite resistance at dc and very small at high frequencies.