Whats use of it? It is required when our circuit includes Push Button.
By reading the Topic of above Post, u might be wondering about what does that whole term Pull up resistor means because you are familiar with the term resistor.
All digital input use Pull up or Pull down resistor to set the default state of the input Pin.
For Example, Consider a circuit containing a Push Button, If the Push Button is closed then it will give HIGH Value But when it is closed it will give any arbitrary value (called floating condition) due to Electrical Noise.
To prevent this from happening, Arduino uses the concept of Pull up Resistor. In Arduino Digital Pin has 10 Kilo ohm resistor connected to it via 5 V to Ground.
During open condition, it will flow through resistance path that is Pull up resistor so, there would be no floating value or value due to electrical noise or anything.
The above fig is an Circuit Diagram of How Pull up resistor are connected. That MCU is Micro controller unit that you can consider as Arduino, that VCC is Power Supply Pin.
There is one more thing like Pull up resistor that is Pull Down resistor, its working is same like Pull Up resistor, There is only one difference that is in Circuit Diagram.
Pull Down resistors are connected to digital Pin via 5 V Source.