Explain trim pot for VU meter
The pot is connected as a rheostat. It will vary from about zero ohms when the wiper is at the top to about 20K when it is at the bottom. 

The diodes are connected as a bridge rectifier so that the current through RV5 passes through the meter in the same direction (pin 2 is positive, pin 1 is negative). 

The meter has some current sensitivity and resistance so it adjusts the input voltage (in this case average AC voltage) that will result in full scale needle deflection.