The capacitors in the base circuit illustrate filtering using capacitive reactance vs frequency responce. The RF is shunted to ground through the .00005 capacitor while the 5 MF capacitor passes the audio to the base. The capacitor opposes circuit voltage change by charging and discharging. The smaller capacitor charges quicker and the can no longer respond. We say they block DC and pass AC and they do. They block DC by charging to the DC level and then they can not pass it. They pass AC only as long as they can charge and discharge.