why is resistor needed for pull up?

Using Arduino Project Guidance
1

Hi,

For the adafruit dht22 sensor it says that you need to place a to place

a 10K resistor between VCC and the data pin, to act as a medium-strength pull up on the data line.

Why is this necessary? and what does the resistor do?

Thanks

2

a signal line that has no signal will act like an antenna.
a signal line that has no signal can float to any voltage
a pull-up or pull-down resistor, is a simple drain to give that line some way for stray voltages to go.

3

If you connect the input to Vcc directly (without a resistor) the line is pulled up to a known voltage, but if a LOW is sent to the input there is a short to ground (Vcc to ground). The resistor prevents the short, limits the current flowing and allows the input to be pulled LOW.

4

You can use the internal pullup and skip external resistors also.
In setup() :
pinMode (pinX, INPUT_PULLUP);

Then in loop(), look for a LOW:

if (digitalRead(pinX) == LOW){
// pin connected to GND with a switch/button. Do the LOW action
}
else{
// still HIGH, ignore or do the HIGH action
}
5

If I may add to CrossRoads post. The internal pullups are, on the mega328, 20K to 50K. They may not be strong enough in some situations, such as a switch with long wires, to overcome the noise picked up. Then you would use external pullups with a lower value (10K to 1K).