What is this?

I'm new to arduino and don't know what the component to the left side of the resistor is. Can someone answer that? Thanks

The image austinbourret intended to embed:

It's a CdS photocell: https://www.adafruit.com/product/161

Do you know why it has to be included in this circuit? Sorry for cross posting, I tried to delete the second post when I realized it was in the wrong section but couldn't for some reason.

It's used to measure light intensity. The resistance decreases with increasing light intensity. In combination with the resistor, it creates a voltage divider. That causes different voltages to be input to pin A0 depending on the light intensity. So if you do something like this:

void setup() {
  Serial.begin(9600);
  while(!Serial){}
}

void loop() {
  Serial.println(analogRead(A0));
  delay(200);
}

You will see a different values printed to the Serial Monitor depending on the light intensity.

The diagram shows an Arduino with two sensors connected. The photocell on Analog 0 measures light intensity, and the TMP36 (?) on Analog 1 measures temperature.

The tutorial https://learn.adafruit.com/tmp36-temperature-sensor/using-a-temp-sensorsays:

This example from the light&temp datalogging tutorial has a photocell but you can ignore it

Ahhh. That makes more sense. The link I was looking at didn't say anything about the use of a light sensor and I thought it had something to do with the circuitry for the temperature sensor. That answers it. Thanks.