analogRead

Analog input: Here we have the ability to measure how much voltage is present at the pin. Not every pin on a typical microprocessor can do this. The voltage must be between zero and the supply voltage (3.3 volts on ESP32). The pin uses a circuit called an ADC or Analog to Digital Converter that "measures" the voltage and converts it into a value . The value produced depends on how many bits the processor uses. An Arduino uses 10 bits (values from 0 to 1023. The ESP32 uses 12 bits (values from 0 to 4095). 1023 is 11 1111 1111 or 10 ones in binary, 4095 is 1111 1111 1111 or 12 ones in binary. Here's a review of binary.
The analogRead(pin) command is used. Like digitalRead, we need to set some other variable equal to the value this command generates. myVoltage = analogRead(pin). We can use this when we want finer control over a process. For instance, we can use a rotary variable resistor and analogRead to control how fast we spin a motor.

In order to try this, we need some voltages to measure. If you have a variable resistor you could follow this lab demo on Random Nerd Tutorials. Your kit does not have a variable resistor (sometimes called 'pot' - short for potentiometer). We can try making a variable resistor using the graphite from a pencil. If we create a thick black line on paper by laying down a thick continuous layer of graphite we might get a very small amount of current to flow through. Graphite has a very high resistance - just be careful that the + and ground leads do not touch directly.

I drew mine like dumbbells' 5 cm long near the fold of a sheet of paper folded in half for strength. I took a jumper from ground and taped the other end to the right hand bell. I took another jumper from 3.3V and taped it to the left hand bell. A third jumper is held in hand and connected to GPIO15. Make sure the 3.3V and ground jumpers do not touch.