The round component with two radial leads is a CdS photocell. This particular photocell is an Advanced Photonix PDV-P8103. A key specification of a photocell is the resistance in darkness vs. bright light. For the PDV-P8103, the dark resistance is specified as minimum 500k ohm and the illuminated resistance is specified between 16k ohm and 33k ohm.
There is also a 10k ohm 1/8 watt resistor with axial leads provided.
A typical means of measuring light with a photocell using a microcontroller is to use the photocell as one resistor in a voltage divider circuit. The voltage divider circuit center point is connected to an analog-to-digital converter (ADC) input of the microcontroller. The ADC measures the voltage at the junction of the two resistors. By calibrating the photocell measurement vs. a known intensity of light, the photocell voltage divider input the ADC can converted to a light level.
A voltage source is needed for the voltage divider circuit, which on the Pi Pico can be taken from the "3V3 OUT" pin.