This circuit will take an input signal and output its highest peak (until the capacitor dies out, so within reason.) The diode only allows a positive voltage through it, so it will restrict any drop in voltage from affecting the capacitor. The capacitor in turn will take in any increase in voltage and hold it - but because no decrease in voltage will reach it, it will theoretically never disharge. In practice, it'll start to drop due to current leakage, but that's negligible (especially with a large enough capacitor or a fast enough signal.)